




















































































THE MOON 

CONSIDERED AS 


A PLANET, A WORLD, AND A SATELLITE 


First Edition 
Second Edition 
Third Edition 
Fourth Edition . 


February 1874 
December 1874 
September 1875 
July 1903 



THE MOON 

CONSIDERED AS A PLANET 
A WORLD, AND A SATELLITE 

BY JAMES NASMYTH, C.E. 

AND 

JAMES CARPENTER, F.R.A.S. 

LATE OF THE ROYAL OBSERVATORY, GREENWICH 


WITH TWENTY-SIX ILLUSTRATIVE PLATES OF LUNAR OBJECTS 
PHENOMENA, AND SCENERY, NUMEROUS DIAGRAMS, ETC. 


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NEW YORK 

JAMES POTT & CO. 

LONDON: JOHN MURRAY 


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HIS GRACE THE DUKE OF ARGYLL 


IN RECOGNITION OF HIS LONG CONTINUED INTEREST IN THE 
SUBJECT OF WHICH IT TREATS, THIS VOLUME IS 
MOST RESPECTFULLY DEDICATED 
BY THE AUTHORS 


viii 


AUTHORS* ORIGINAL PREFACE 


cause of volcanic energy and the mode of its action 
as manifested in the characteristic craters and other 
eruptive phenomena that abound upon the moon's 
surface. We have endeavoured to explain these 
phenomena by reference to a few natural laws, and 
to connect them with the general hypothesis of 
planet formation which is now widely accepted by 
cosmologists. The principal aim of our work is to 
lay these proffered explanations before the students 
and admirers of astronomy and science in general; 
and we trust that what we have deduced concern¬ 
ing the moon may be taken as referring to a certain 
extent to other planets. 

Some reflections upon the moon considered as a 
world, in reference to questions of habitability, and 
to the peculiar conditions which would attend a 
sojourn on the lunar surface, have appeared to us 
not inappropriate. These, though instructive, are 
rather curious than important. More worthy of 
respectful consideration are the few remarks we 
have offered upon the moon as a satellite and a 
benefactor to the inhabitants of this Earth. 

In reference to the illustrations accompanying 
this work, more especially those which represent 


AUTHORS 5 ORIGINAL PREFACE 


IX 


certain portions of the lunar surface as they are 
revealed by the aid of powerful telescopes, such as 
those which we employed in our scrutiny, it is 
proper that we should say a few words here on the 
means by which they have been produced. 

During upwards of thirty years of assiduous 
observation, every favourable opportunity has been 
seized to educate the eye, not only in respect to 
comprehending the general character of the moon’s 
surface, but also to examining minutely its marvel¬ 
lous details under every variety of phase, in the 
hope of rightly understanding their true nature, as 
well as the causes which had produced them. This 
object was aided by making careful drawings of 
each portion or object when it was most favour¬ 
ably presented in the telescope. These drawings 
were again and again repeated, revised, and com¬ 
pared with the actual objects, the eye thus advanc¬ 
ing in correctness and power of appreciating minute 
details, while the hand was acquiring, by assiduous 
practice, the art of rendering correct representations 
of the objects in view. In order to present these 
illustrations with as near an approach as possible 
to the absolute integrity of the original objects, the 
idea occurred to us that by translating the drawings 


X 


AUTHORS’ ORIGINAL PREFACE 


into models which, when placed in the sun’s rays, 
would faithfully reproduce the lunar effects of light 
and shadow, and then photographing the models so 
treated, we should produce most faithful representa¬ 
tions of the original. The result was in every way 
highly satisfactory, and has yielded pictures of the 
details of the lunar surface such as we feel every 
confidence in submitting to those of our readers 
who have made a special study of the subject. It 
is hoped that those also who have not had oppor¬ 
tunity to become intimately acquainted with the 
details of the lunar surface, will be enabled to 
become so by aid of these illustrations. 

In conclusion, we think it desirable to add that 
the photographic illustrations above referred to are 
printed by well-established pigment processes which 
ensure their entire permanency. 


PUBLISHER’S NOTE TO THE FOURTH 
EDITION 

The first three editions of this work have been out 
of print for some years; but as the work has been 
very well received by those who are specially quali¬ 
fied to judge of its value, and as enquiries for copies 
continue to be made, we have been induced to bring 
out a new and popular edition, in a compact size 
and at a greatly reduced price, and it is hoped that 
these qualifications may bring this valuable and 
unique book within the reach of many who have 
hitherto been unable to obtain it. 


May 1903. 

































































. 




V.'.-'A- 



' 






CONTENTS 

CHAPTER I 

ON THE COSMICAL ORIGIN OF THE PLANETS OF THE 
SOLAR SYSTEM 

PAGE 

Origination of Material Things—Celestial Vapours—Nebulae— 

Their vast Numbers—Sir W. Herschel’s Observations and 
Classification — Buffon’s Cosmogony — Laplace’s Nebular 
Hypothesis—Doubts upon its Validity—Support from Spec¬ 
trum Analysis ^ . 1 

CHAPTER II 

THE GENERATION OF COSMICAL HEAT 

Conservation of Force—Indestructibility of Force—Its Converti¬ 
bility into Heat—Dawn of the Doctrine—Mayer’s Deductions 
—Joule’s Experiments—Mechanical Equivalent of Heat— 
Gravitation the Source of Cosmical Heat—Calculations of 
Mayer and Helmholtz—The Moon as an Incandescent Sphere 
—Not necessarily Burning—Loss of Heat by Radiation— 
Cooling of External Crust—Commencement of Selenological 
History . . . . . . . .20 


CHAPTER III 

THE SUBSEQUENT COOLING OF THE IGNEOUS BODY 

Cooling commenced from Outer Surface—Contraction by Cooling— 
Expansion of Molten Matter upon Solidification—Water not 
exceptional—Similar Behaviour of Molten Iron—Floating of 
Solid on Molten Metal—Currents in a Pot of Molten Metal 
—Bursting of Iron Bottle by Congelation of Bismuth within 
—Evidence from Furnace Slag—From the Crater of Vesuvius 
—Effects of Contraction of Moon’s Crust and Expansion of 
Interior—Production of Ridges and Wrinkles—Theory of 
Wrinkles—Example from Shrivelled Apple and Hand 

xiii 


34 


XVI 


CONTENTS 


CHAPTER IX 

ON THE GREAT RING-FORMATIONS NOT MANIFESTLY 
VOLCANIC 

PAGE 

Absence of Central Cones—Vast Diameters—Difficult of Explana¬ 
tion—Hooke’s Idea—Suggested Cause of True Circularity— 
Scrope’s Hypothesis of Terrestrial Tumescences — Rozet’s 
Tourbillonic Theory—Dana’s Ebullition Theory . .193 


CHAPTER X 

PEAKS AND MOUNTAIN RANGES 

Paucity of extensive Mountain Systems on Moon—Contrast with 
Earth—Lunar Mountains found in less disturbed Regions— 

Lunar Apennines, Caucasus, and Alps—Valley of Alps— 
“Crag and Tail’’Contour—Isolated Peaks—How Produced 
—Analogy from Freezing Fountain—Terrestrial Counterparts 
and their Explanation by Scrope—Blowing Cone on Teneriffe 
—Comparative Gentleness of Mountain-forming Action— 
Relation between Mountain Systems and Crater Systems— 
Wrinkle Ridges ....... 203 


CHAPTER XI 

CRACKS AND RADIATING STREAKS 

Description—Divergence from Focal Craters—Experimental Ex¬ 
planation of their Cause—Radial Cracking of Crust—Outflow 
of Matter therefrom—Analogy from “Starred” Ice—No 
Shadows cast by Streaks—Their probable Slight Elevation— 

Open Cracks—Great Numbers—Length—Depth—In-fallen 
Fragments—Shrinkage a Cause of Cracks—Lateness of their 
Production ....... 216 


CONTENTS 


xvn 


CHAPTER XII 

COLOUR AND BRIGHTNESS OF LUNAR DETAILS: 
CHRONOLOGY OF FORMATIONS, AND FINALITY 
OF EXISTING FEATURES 

PAGE 

Absence of Conspicuous Colour—Slight tints of “ Seas ”—Cause— 
Probable Variety of Tints in small Patches—Diversity of 
Brightness of Details—Most Conspicuous at Full Moon— 
Classification of Shades—Exaggerated Contrasts in Photo¬ 
graphs— Brightest Portions probably the latest formed — 
Chronology of Formations—Large Craters older than Small— 
Mountains older than Craters—Bright Streaks comparatively 
recent — Cracks most recent of all features — Question of 
existing Change—Evidence from Observation—Paucity of 
such Evidence—Supposed Case of Linn<Z —Theoretical Dis¬ 
cussion—Relative Cooling Tendencies of Earth and Moon— 

Earth nearly assumed its Final Condition—Moon probably 
cooled Ages upon Ages ago—Possible slight Changes from 
Solar Heating—Disintegrating Action . . . 230 


CHAPTER XIII 

THE MOON AS A WORLD: DAY AND NIGHT UPON 
ITS SURFACE 

Existence of Habitants on other Planets—Interest of the Question 
—Conditions of Life—Absence of these from Moon—No Air 
or Water and intense Heat and Cold—Possible Existence of 
Protogerms of Life—A Day on the Moon imagined—In¬ 
structiveness of the Realisation—Length of Lunar Day— 

No Dawn or Twilight—Sudden Appearance of Light—Slow¬ 
ness of Sun in Rising—No Atmospheric Tints—Blackness of 
Sky and Visibility of Stars and Fainter Luminosities at 
Noon-Day—Appearance of the Earth as a Stationary Moon 
—Its Phases—Eclipse of Sun by Earth—Attendant Pheno¬ 
mena—Lunar Landscape—Height essential to secure a Point 
of View—Sunrise on a Crater — Desolation of Scene — No 
Vestige of Life—Colour of Volcanic Products—No Atmos¬ 
pheric Perspective—Blackness of Shadows—Impressions on 


CONTENTS 


xviii 

PAGE 

other Senses than Light—Heat of Sun untempered—Intense 
Cold in Shade—Dead Silence—No Medium to conduct Sound 
—Lunar Afternoon and Sunset—Night—The Earth a Moon 
—Its Size, Rotation, and Features—Shadow of Moon upon 
it—Lunar Night-Sky—Constellations—Comets and Planets 
— No Visible Meteors — Bombardment by Dark Meteoric 
Masses—Lunar Landscape by Night—Intensity of Cold . 252 


CHAPTER XIV 

THE MOON AS A SATELLITE : ITS RELATION TO THE 
EARTH AND MAN 

The Moon as a Luminary — Secondary Nature of Light-giving 
Function—Primary Office as a Sanitary Agent—Cleansing 
Effects of the Tides—Tidal Rivers and Transport thereby— 

The Moon a “Tug”—Available Power of Tides—Tide-Mills— 
Transfer of Tidal Power Inland—The Moon as a Navigator’s 
Guide—Longitude found by the Moon—Moon’s Motions— 
Discovered by Observations—Grouped into Theories—Repre¬ 
sented by Tables—The Nautical Almanac—The Moon as a 
Long-period Timekeeper — Reckoning by “ Moons ” — 
Eclipses the Starting-points of Chronologies—Furnish indis¬ 
putable Dates—Solar Surroundings revealed by Eclipses 
when Moon screens the Sun — Solar Corona — Moon as 
a Medal of Creation, a Half-formed World—Abuses of the 
Moon—Superstitions—Erroneous Ideas regarding Moonlight 
betrayed by Artists and Authors — The Moon and the 
Weather—Errors and Facts—Atmospheric Tides—Warmth 
from Moon—Paradoxical Effect in cooling the Earth . 282 

CHAPTER XV 

CONCLUDING SUMMARY.306 


PLATE 


LIST OF PLATES 


PAGE 


Aspect op an Eclipse op the Sun as it would 


appear as seen FROM the Moon . Frontispiece. 


I. 

Crater op Vesuvius, 1864 

To face page 

44 

II. 

Back op Hand \ 


AQ . 

III. 

Shrivelled Apple / 


*±o 

IY. 

Full Moon .... 

To face page 

86 

Y. 

Picture Map of the Moon, with accompanying 



Key Plate .... 

. 

119 

YI. 

Copernicus 

To face page 

122 

VII. 

Gassendi .... 

» 

126 

VIII. 

Aristotle and Eudoxus 

• 

128 

IX. 

Triesnecker .... 


130 

X. 

Theophilus, Cyrillus, and Catharina 


132 

XI. 

Ptolemy, Alphons, Arzachael, etc. . 


134 

XII. 

Plato, the Valley op the Alps, Pico, 

ETC. 

136 

XIII. 

Tycho and its Surroundings 

. 

140 

XIY. 

Aristarchus and Herodotus. 


144 

XY. 

Vesuvius and Neighbourhood 




op Naples . . . r . 


154 

XYI. 

Portion of the Moon’s Surface I 



XVII. 

Normal Lunar Crater 


170 

XVIII. 

Wargentin .... 

To face page 

176 

XIX. 

Overlapping Craters . 


188 

XX. 

Mercator and Campanus 


202 

XXI. 

Ideal Sketch op Pico . 


206 

XXII. 

The Lunar Apennines, Archimedes, etc. . 

210 

XXIII. 

Glass Globe, Cracked by i 




Internal Pressure . . 1 To face each other 

218 

XXIV. 

Full Moon . . . J 



XXY. 

Group op Lunar Mountains . 


272 





f 













THE MOON 


CHAPTEE I 

ON THE COSMICAL ORIGIN OF THE PLANETS 
OF THE SOLAR SYSTEM 

In this chapter we propose to treat briefly of the 
probable formation of the various members of the 
solar system from matter which previously existed 
in space in a condition different from that in which 
we at present find it— i.e., in the form of planets 
and satellites. 

It is almost impossible to conceive that our 
world with its satellite, and its fellow worlds with 
their satellites, and also the great centre of them 
all, have always, from the commencement of time, 
possessed their present form: all our experiences 
of the working of natural laws rebel against such 
a supposition. In every phenomenon of nature 
upon this earth—the great field from which we 
must glean our experiences and form our analogies 

A 


2 


COSMICAL ORIGIN OF PLANETS [chap. 


—we see a constant succession of changes going on, 
a constant progression from one stage of develop¬ 
ment to another taking place, a perpetual mutation 
of form and nature of the same material substance 
occurring: we see the seed transformed into the 
plant, the flower into the fruit, and the ovum into 
the animal. In the inorganic world we witness the 
operation of the same principle; but, by reason of 
their slower rate of progression, the changes there 
are manifested to us rather by their resulting effects 
than by their visible course of operation. And 
when we consider, as we are obliged to do, that 
the same laws work in the greatest as well as the 
smallest processes of nature, we are compelled to 
believe in an antecedent state of existence of the 
matter that composes the host of heavenly bodies, 
and amongst them the earth and its attendant 
moon. 

In the pursuit of this course of argument we 
are led to inquire whether there exists in the 
universe any matter from which planetary bodies 
could be formed, and how far their formation from 
such matter can be explained by the operation of 
known material laws. 

Before the telescope revealed the hidden 
wonders of the skies, and brought its rich fruits into 


i.] FIRST DISCOVERIES OF NEBULAE 3 

our garner of knowledge concerning the nature of 
the universe, the philosophic minds of some early 
astronomers, Kepler and Tycho Brahe to wit, enter¬ 
tained the idea that the sun and the stars—the suns 
of distant systems—were formed by the condensa¬ 
tion of celestial vapours into spherical bodies; Kepler 
basing his opinion on the phenomena of the sudden 
shining forth of new stars on the margin of the 
Milky Way. But it was when the telescope pierced 
into the depths of celestial space, and brought to 
light the host of those marvellous objects, the 
nebulae, that the strongest evidence was afforded 
of the probable validity of these suppositions. The 
mention of “ nebulous stars ” made by the earlier 
astronomers refers only to clusters of telescopic 
stars which the naked eye perceives as small 
patches of nebulous light; and it does not appear 
that even the nebula in Andromeda, although so 
plainly discernible as to be often now-a-days mis¬ 
taken by the uninitiated for a comet, was known, 
until it was discovered by means of a telescope, in 
1612, by Simon Marius, who described it as resem¬ 
bling a candle shining through semi-transparent 
horn, as in a lantern, and without any appearance 
of stars. Forty years after this date Huygens dis¬ 
covered the splendid nebula in the sword handle 


4 


COSMICAL ORIGIN OF PLANETS [chap. 


of Orion, and in 1665 another was detected by 
Hevelius. In 1667 Halley (afterwards Astronomer 
Royal) discovered a fourth; a fifth was found by 
Kirseh in 1681, and a sixth by. Halley again in 
1714. Half a century after this the labours of 
Messier expanded the list of known nebulae and 
clusters to 103, a catalogue of which appeared in 
the Connaissance du Temps (the French Nautical 
Almanac) for the years 1783-1784. But this branch 
of celestial discovery achieved its most brilliant 
results when the rare penetration, the indomitable 
perseverance, and the powerful instruments of the 
elder Herschel were brought to bear upon it. In 
the year 1779 this great astronomer began to search 
after nebulae with a seven-inch reflector, which he 
subsequently superseded by the great one of forty 
feet focus and four feet aperture. In 1786 he 
published his first catalogue of 1000 nebulae; three 
years later he astonished the learned world by a 
second catalogue containing 1000 more, and in 
1802 a third came forth comprising other 500, 
making 2500 in all! This number has been so far 
increased by the labours of more recent astrono¬ 
mers that the last complete catalogue, that of Sir 
John Herschel, published a few years ago, contains 
the places of 5063 nebulae and clusters. 


i] HERSCHEL’S CLASSIFICATION 5 

At the earlier periods of HerscheFs observations, 
that illustrious observer appears to have inclined to 
the belief that all nebulae were but remote clusters 
of stars, so distant, so faint, and so thickly agglom¬ 
erated as to affect the eye only by their combined 
luminosity, and at this period of the nebular history 
it was supposed that increased telescopic power 
would resolve them into their component stars. 
But the familiarity which Herschel gained with 
the phases of the multitudinous nebulae that passed 
in review before his eyes, led him ultimately to 
adopt the opinion, advanced by previous philoso¬ 
phers, that they were composed of some vapoury 
or elementary matter out of which, by the process 
of condensation, the heavenly bodies were formed ; 
and this led him to attempt a classification of the 
known nebulae into a cosmical arrangement, in 
which, regarding a chaotic mass of vapoury matter 
as the primordial state of existence, he arranged 
them into a series of stages of progressive develop¬ 
ment, the individuals of one class being so nearly 
allied to those in the next that, to use his own 
expression, not so much difference existed between 
them “ as there would be in an annual description 
of the human figure were it given from the birth 
of a child till he comes to be a man in his prime.” 


6 COSMICAL ORIGIN OF PLANETS [chap. 

(Philosophical Transactions, vol. ci., pp. 271, et 
seq.) 

His category comprises upwards of thirty classes 
or stages of progression, the titles of a few of which 
we insert here to illustrate the completeness of his 
scheme. 


Class 1. 




6 . 


„ 15- 
17. 
20 . 


JJ 




25. 

29. 






30. 

33. 


Of extensive diffused nebulosity. (A 
table of 52 patches of such nebulosity 
actually observed is given, some of 
which extend over an area of five or 
six square degrees, and one of which 
occupies nine square degrees.) 

Of milky nebulosity with condensation. 

Of nebulae that are of an irregular figure. 

Of round nebulae. 

Of nebulae that are gradually brighter 
in the middle. 

Of nebulae that have a nucleus. 

Of nebulae that draw progressively to¬ 
wards a period of final condensation. 

Of planetary nebulae. 

Of stellar nebulae nearly approaching 
the appearance of stars. 


In a walk through a forest we see trees in every 


I.] 


DEVELOPMENT OF NEBULAE 


7 


stage of growth, from the tiny sapling to the giant 
of the woods, and no doubt can exist in our minds 
that the latter has sprung from the former. We 
cannot at a passing glance discern the process of 
development actually going on; to satisfy ourselves 
of this, we must record the appearance of some 
single tree from time to time through a long series 
of years. And what a walk through a forest is to 
an observer of the growth of a tree, a lifetime is 
to the observer of changes in such objects as the 
nebulae. The transition from one state to another 
of the nebulous development is so slow that a life¬ 
time is hardly sufficient to detect it. Nor can any 
precise evidence of change be obtained by the com¬ 
parison of drawings or descriptions of nebulae at 
various epochs, with whatever care or skill such 
drawings be made, for it will be admitted that no 
two draughtsmen will produce each a drawing of 
the most simple object from the same point of view, 
in which every detail in the one will coincide exactly 
with every detail in the other. There is abundant 
evidence of this in the existing representations of 
the great nebula in Orion; a comparison of the 
drawings that have been lately made of this object, 
with the most perfect instruments and by the most 
skilful of astronomical draughtsmen, reveals varieties 


8 


COSMICAL ORIGIN OF PLANETS [chap. 


of detail and even of general appearance such as 
could hardly be imagined to occur in similar delinea¬ 
tions of one and the same subject; and any one who 
himself makes a perfectly unbiassed drawing at the 
telescope will find upon comparison of it with others 
that it will offer many points of difference. The 
fact is that the drawing of a man, like his penman¬ 
ship, is a personal characteristic, peculiar to himself, 
and the drawings of two persons cannot be expected 
to coincide any more than their handwritings. The 
appearance of a nebula varies also to a great extent 
with the power of the telescope used to observe it and 
the conditions under which it is observed; the draw¬ 
ings of nebulae made with the inferior telescopes of 
a century or two centuries ago, the only ones that, 
by comparison with those made in modern times, 
could give satisfactory evidence of changes of form 
or detail, are so rude and imperfect as to be useless 
for the purpose, and it is reasonable to suppose that 
those made in the present day will be similarly use¬ 
less a century or two hence. Since then we can 
obtain no evidence of the changes we must assume 
these mysterious objects to be undergoing, ipso 
facto , by observation of one nebula at various 
periods , we must for the present accept the primd 
facie evidence offered (as in the case of the trees 


LAPLACE 


9 


iJ 

in a forest) by the observation of various nebulae at 
one 'period. 

“The total dissimilitude,” says Herscliel at the 
close of the observations we have alluded to, 
“between the appearance of a diffusion of the 
nebulous matter and of a star, is so striking, that 
an idea of the conversion of the one into the other 
can hardly occur to any one who has not before him 
the result of the critical examination of the nebulous 
system which has been displayed in this [his] paper. 
The end I have had in view, by arranging my obser¬ 
vations in the order in which they have been placed, 
has been to show that the above-mentioned extremes 
may be connected by such nearly allied intermediate 
steps, as will make it highly probable that every 
succeeding state of the nebulous matter is the result 
of the action of gravitation upon it while in a fore¬ 
going one, and by such steps the successive conden¬ 
sation of it has been brought up to the planetary 
condition. From this the transit to the stellar form, 
it has been shown, requires but a very small addi¬ 
tional compression of the nebulous matter.” 

Where the researches of Herschel terminated 
those of Laplace commenced. Herschel showed 
how a mass of nebulous matter so diffused as to be 
scarcely discernible might be and probably was, by 


10 COSMICAL ORIGIN OF PLANETS [chap. 

the mere action of gravitation, condensed into a mass 
of comparatively small dimensions when viewed in 
relation to the immensity of its primordial condition. 
Laplace demonstrated how the known laws of gravi¬ 
tation could and probably did from such a partially 
condensed mass of matter produce an entire planet¬ 
ary system with all its subordinate satellites. 

The first physicist who ventured to account for 
the formation of the various bodies of our solar 
system was Buffon, the celebrated French naturalist. 
His theory, which is fully detailed in his renowned 
work on natural history, supposed that at some 
period of remote antiquity the sun existed without 
any attendant planets, and that a comet having 
dashed obliquely against it, ploughed up and drove 
off a portion of its body sufficient in bulk to form 
the various planets of our system. He suggests 
that the matter thus carried off “ at first formed a 
torrent the grosser and less dense parts of which 
were driven the farthest, and the densest parts, 
having received only the like impulsion, were not 
so remotely removed, the force of the sun’s attrac¬ 
tion having retained them: ” that “ the earth and 
planets therefore at the time of their quitting the 
sun were burning and in a state of liquefaction; ” 
that “by degrees they cooled, and in this state of 


BUFFON’S HYPOTHESIS 


11 


i-J 

fluidity they took their form,” He goes on to say 
that the obliquity of the stroke of the comet might 
have been such as to separate from the bodies of 
the principal planets small portions of matter, which 
would preserve the same direction of motion as the 
principal planets, and thus would form their attend¬ 
ant satellites. 

The hypothesis »of Buffon, however, is not suffi¬ 
cient to explain all the phenomena of the planetary 
system; and it is imperfect, inasmuch as it begins 
by assuming the sun to be already existing, whereas 
any theory accounting for the primary formation of 
the solar system ought necessarily to include the 
origination of the most important body thereof, the 
sun itself. Nevertheless, it is but due to Buffon to 
mention his ideas, for the errors of one philosophy 
serve a most useful end by opening out fields of 
inquiry for subsequent and more fortunate specu¬ 
lators. 

Laplace, dissatisfied with Buffon’s theory, sought 
one more probable, and thus was led to the pro¬ 
position of the celebrated nebular hypothesis which 
bears his name, and which, in spite of its dis¬ 
believers, has never been overthrown, but remains 
the only probable, and, with our present knowledge, 
the only possible explanation of the cosmical origin 


12 COSMICAL ORIGIN OF PLANETS [chap. 

of the planets of our system. Although Laplace 
puts forth his conjectures, to use his own words, 
“ with the deference which ought to inspire every¬ 
thing that is not a result of observation and calcula¬ 
tion,” yet the striking coincidence of all the planet¬ 
ary phenomena with the conditions of his system 
gives to those conjectures, again to use his modest 
language, “ a probability strongly approaching certi¬ 
tude.” 

Laplace conceived the sun to have been at one 
period the nucleus of a vast nebula, the attenuated 
surrounding matter of which extended beyond what 
is now the orbit of the remotest planet of the 
system. He supposed that this mass of matter in 
process of condensation possessed a rotatory motion 
round its centre of gravity, and that the parts of it 
that were situated at the limits where centrifugal 
force exactly counterbalanced the attractive force 
of the nucleus were abandoned by the contracting 
mass, and thus were formed successively a number 
of rings of matter concentric with and circulating 
around the central nucleus. As it would be impro¬ 
bable that all the conditions necessary to preserve 
the stability of such rings of matter in their annular 
form could in all cases exist, they would break up 
into masses which would be endued with a motion 


i] LAPLACE’S NEBULAR HYPOTHESIS 13 

of rotation, and would in consequence assume a 
spheroidal form. These masses, which hence con¬ 
stituted the various planets, in their turn condens¬ 
ing, after the manner of the parent mass, and 
abandoning their outlying matter, would become 
surrounded by similarly concentric rings, which 
would break up and form the satellites surround¬ 
ing the various planetary masses; and, as a remark¬ 
able exception to the rule of the instability of the 
rings and their consequent breakage, Laplace cited 
the case of Saturn surrounded by his rings as the 
only instances of unbroken rings that the whole 
system offers us; unless, indeed, we include the 
zodiacal light, that cone of hazy luminosity that is 
frequently seen streaming from our luminary shortly 
before and after sunset, and which Laplace supposed 
to be formed of molecules of matter, too volatile to 
unite either with themselves or with the planets, 
and which must hence circulate about the sun in 
the form of a nebulous ring, and with such an 
appearance as the zodiacal actually presents. 

This hypothesis, although it could not well be 
refuted, has been by many hesitatingly received, 
and for a reason which was at one time cogent. 
In the earlier stages of nebular research it was 
clearly seen, as we have previously remarked, that 


14 COSMICAL ORIGIN OF PLANETS [chap. 

many of the so-called nebulae, which appeared at 
first to consist of masses of vapoury matter, became, 
when scrutinised with telescopes of higher power, 
resolved into clusters containing countless numbers 
of stars, so small and so closely agglomerated, that 
their united lustre only impressed the more feeble 
eye as a faint nebulosity; and as it was found that 
each accession of telescopic power increased the 
numbers of nebulae that were thus resolved, it was 
thought that every nebula would at some period 
succumb to the greater penetration of more power¬ 
ful instruments; and if this were the case, and if 
no real nebulae were hence found to exist, how, it 
was argued, could the nebular hypothesis be main¬ 
tained ? One of the most important nebulae bear¬ 
ing upon this question was the great one in the 
sword handle of Orion, one of the grandest and 
most conspicuous in the whole heavens. On 
account of the brightness of some portions of this 
object, it seemed as though it ought to be readily 
resolvable, supposing all nebulae to consist of stars, 
but all attempts to resolve it were in vain, even 
with the powerful telescopes of Sir John Herschel 
and the clear zenethal sky of the Cape of Good 
Hope. At length the question was thought to be 
settled, for upon the completion of Lord Rosse’s 


THE SPECTROSCOPE 


15 


ij 

giant reflector, and upon examination of the nebula 
with it, his lordship stated that there could be 
little, if any, doubt as to its resolvability, and then 
it was maintained, by the disbelievers in the nebular 
theory, that the last stronghold of that theory had 
been broken down. 

But the truths of nature are for ever playing at 
hide and seek with those who follow them:—the 
dogmas of one era are the exploded errors of the 
next. Within the past few years a new science 
has arisen that furnishes us with fresh powers of 
penetration into the vast and secret laboratories of 
the universe; a new eye, so to speak, has been 
given us by which we may discern, by the mere 
light that emanates from a celestial body, some¬ 
thing of the chemical elements of which it is com¬ 
posed. When Newton two hundred years ago 
toyed with the prism he bought at Stourbridge fair, 
and projected its pretty rainbow tints upon the wall, 
his great mind little suspected that that phantom 
riband of gorgeous colours would one day be called 
upon to give evidence upon the probable cosmical 
origin of worlds. Yet such in truth has been the 
case. Every substance when rendered luminous 
gives off light of some colour or degree of refrangi- 
bility peculiar to itself, and although the eye cannot 


16 


COSMICAL ORIGIN OF PLANETS [chap. 


detect any difference between one character of light 
and another, the prism gives the means of ascertain¬ 
ing the quality and degree of refrangibility of the 
light emanating from any source however distant, 
and hence of gaining some knowledge of the nature 
of that source. If, for instance, a ray of light from 
a solid body in combustion is passed through a 
prism, a spectrum is produced which exhibits light 
of all colours or all degrees of refrangibility; if the 
light from such a body, before passing through the 
prism, be made to pass through gases or certain 
metallic vapours, the resulting spectrum is found 
to be crossed transversely by numbers of fine dark 
lines, apparently separating the various colours, or 
cutting the spectrum into bands. The solar spec¬ 
trum is of this class; the once mysterious lines 
first observed by Wollaston, and subsequently by 
Fraunhofer, and known as “ Fraunhofer’s lines,” 
have now been interpreted, chiefly by the sagacious 
German chemist Kirchhoff, and identified as the 
effects of absorption of certain of the sun’s rays by 
chemical vapours contained in his atmosphere. The 
fixed stars yield spectra of the same character, but 
varying considerably in feature, the lines crossing the 
Stella spectra differing in position and number from 
those of the sun, and one star from another, prov- 


i ] SIR W. HUGGINS* DISCOVERY 17 

mg the stars to possess varied chemical constitu¬ 
tions. But there is another class of spectra, ex¬ 
hibited when light from other sources is passed 
through the prism. These consist, not of a lumin¬ 
ous riband of light like the solar spectrum, but of 
bright isolated lines of coloured light with com¬ 
paratively wide dark spaces separating them. Such 
spectra are yielded only by the light emitted from 
luminous gases and metals or chemical elements in 
the condition of incandescent vapour. Every gas 
or element in the state of luminous vapour yields a 
spectrum peculiar to itself, and no two elements 
when vaporised before the prism show the same 
combinations of luminous lines. 

Now in the course of some observations upon 
the spectra of the fixed stars by Dr Huggins, # it 
occurred to that gentleman to turn his telescope, 
armed with a spectroscope, upon some of the brighter 
of the nebulae, and great was his surprise to find 
that instead of yielding continuous spectra, as they 
must have done had their light been made up of 
that of a multitude of stars, they gave spectra 
containing only two or three isolated bright lines; 
such a spectrum could only be produced by some 
luminous gas or vapour, and of this form of matter 

* Dr Huggins became in 1897 Sir William Huggins, K.C.B. 

B 


18 


COSMICAL ORIGIN OF PLANETS [chap. 


we are now justified in declaring, upon the strength 
of numerous modern observations, these remark¬ 
able bodies are composed; and it is a curious and 
interesting fact that some of the nebulae styled 
resolvable, from the fact of their exhibiting points 
of light like stars, yield these gaseous spectra, 
whence Dr Huggins concludes that the brighter 
points taken for stars are in reality nuclei of greater 
condensation of the nebular matter: and so the 
fact of the apparent resolvability of a nebula affords 
no positive proof of its non-nebulous character. 

These observations—which have been fully con¬ 
firmed by Father Secchi of the Roman College—by 
destroying the evidence in favour of nebulae being 
remote clusters, add another attestation to the 
probability of the truth of the nebular hypothesis, 
and we have now the confutation of the lumin- 
ologist to add to that of the astronomers who, in 
the person of the illustrious Arago, asserted that 
the ideas of the great author of the Mecanique 
Celeste “ were those only which by their grandeur, 
their coherence, and their mathematical character 
could be truly considered as forming a physical 
cosmogony.” 

Confining, then, our attention to the single 
object of the universe it is our task to treat of— 


i.] THE MOON ONCE DIFFUSED MATTER 19 


the Moon—and without asserting as an indisput¬ 
able fact that which we can never hope to know 
otherwise than by inference and analogy, we may 
assume that that body once existed in the form of 
a vast mass of diffused or attenuated matter, and 
that, by the action of gravitation upon the particles 
of that matter, it was condensed into a compara¬ 
tively small and compact planetary body. 

But while the process of condensation or com¬ 
paction was going on, another important law of 
nature—but recently unfolded to our knowledge— 
was in powerful operation, the discussion of which 
law we reserve for a separate chapter. 


CHAPTER II 


THE GENERATION OF COSMICAL HEAT 

In the preceding chapter we endeavoured to show 
how the action of gravitation upon the particles of 
diffused primordial matter would result in the 
formation, by condensation and aggregation, of a 
spherical planetary body. We have now to con¬ 
sider another result of the gravitating action, and 
for this we must call to our aid a branch of 
scientific inquiry and investigation unrecognised as 
such at the period of Laplace’s speculations, and 
which has been developed almost entirely within 
the past quarter of a century. 

The “great philosophical doctrine of the 
present era of science,” as the subject about to 
engage our attention has been justly termed, bears 
the title of the “Conservation of Force,” or—as 
some ambiguity is likely to attend the definition of 
the term “Force”—the “Conservation of Energy.” 


chap, ii.] CONSERVATION OF ENERGY 


21 


The basis of the doctrine is the broad and compre¬ 
hensive natural law which teaches us that the 
quantity of force comprised by the universe, like 
the quantity of matter contained in it, is a fixed 
and invariable amount, which can be neither added 
to nor taken from, but which is for ever under¬ 
going change and transformation from one form to 
another. That we cannot create force ought to be 
as obvious a fact as that we cannot create matter; 
and what we cannot create we cannot destroy. As 
in the universe we see no new matter created, but 
the same matter constantly disappearing from one 
form and reappearing in another, so we can find no 
new force ever coming into action—no description 
of force that is not to be referred to some previous 
manner of existence. 

Without entering upon a metaphysical discus¬ 
sion of the term “force,” it will be sufficient for 
our purpose to consider it as something which pro¬ 
duces or resists motion, and hence we may argue 
that the ultimate effect of force is motion. The 
force of gravity on the earth results in the motion 
or tendency of all bodies towards its centre, and 
similarly, the action of gravitation upon the atoms 
or particles of a primeval planet resulted in the 
motion of those particles towards each other. We 


22 GENERATION OF COSMICAL HEAT [chap. 

cannot conceive force otherwise than by its effects, 
or the motion it produces. 

And force we are taught is indestructible; there¬ 
fore motion must be indestructible also. But when 
a falling body strikes the earth, or a gunshot strikes 
its target, or a hammer delivers a blow upon an 
anvil, or a brake is pressed against a rotating 
wheel, motion is arrested, and it would seem 
natural to infer that it is destroyed. But if we 
say it is indestructible, what becomes of it ? The 
philosophical answer to the question is this—that 
the motion of the mass becomes transferred to the 
particles or molecules composing it, and transformed 
to molecular motion, and this molecular motion 
manifests itself to us as heat. The particles or 
atoms of matter are held together by cohesion, or, 
in other words, by the action of molecular attrac¬ 
tion. When heat is applied to these particles, 
motion is set up among them, they are set in 
vibration, and thus, requiring and making wider 
room, they urge each other apart, and the well- 
known expansion by heat is the result. If the heat 
be further continued a more violent molecular 
motion ensues, every increase of heat tending to 
urge the atoms further apart, till at length they 
overcome their cohesive attraction and move about 


II.] 


HEAT A MODE OF MOTION 


23 


each other, and a liquid or molten condition results. 
If the heat be still further increased, the atoms 
break away from their cohesive fetters altogether 
and leap off the mass in the form of vapour, and 
the matter thus assumes the gaseous or vaporous 
form. Thus we see that the phenomena of heat 
are phenomena of motion, and of motion only. 

This mutual relation between heat and work 
presented itself as an embryo idea to the minds 
of several of the earlier philosophers, by whom it 
was maintained in opposition to the material theory 
which held heat to be a kind of matter or subtle 
fluid stored up in the inter-atomic spaces of all 
bodies, capable of being separated and procured 
from them by rubbing them together, but not gener¬ 
ated thereby. Bacon, in his Novum Organum , says 
that “heat itself, its essence and quiddity, is motion 
and nothing else.” Locke defines heat as “a very 
brisk agitation of the insensible parts of an object, 
which produces in us that sensation from whence we 
denominate the object hot; so what in our sensation 
is heat , in the object is nothing but motion .” Des¬ 
cartes and his followers upheld a similar opinion. 
Richard Boyle, two hundred years ago, actually 
wrote a treatise entitled “ The Mechanical Theory 
of Heat and Cold,” and the ingenious Count Rum- 


24 


GENERATION OF COSMICAL HEAT [chap. 


ford made some highly interesting and significant 
experiments on the subject, which are described in 
a paper read before the Royal Society in 1798, 
entitled “ An Inquiry concerning the Source of Heat 
excited by Friction.” But the conceptions of these 
authors remained isolated and unfruitful for more 
than a century, and might have passed, meantime, 
into the oblivion of barren speculation, but for the 
impulse which this branch of inquiry has lately 
received. Now, however, they stand forth as not¬ 
able instances of truth trying to force itself into 
recognition while yet men’s minds were unprepared 
or disinclined to receive it. The key to the beauti¬ 
ful mechanical theory of heat was found by these 
searching minds, but the unclasping of the lock 
that should disclose its beauty and value was 
reserved for the philosophers of the present age. 

Simultaneously and independently, and without 
even the knowledge of each other, three men, far 
removed from probable intercourse, conceived the 
same ideas and worked out nearly similar results 
concerning the mechanical theory of heat. Seeing 
that motion was convertible into heat, and heat 
into motion, it became of the utmost importance to 
determine the exact relation that existed between 
the two elements. The first who raised the idea to 


II.] 


THREE DISCOVERERS 


25 


philosophic clearness was Dr Julius Robert Mayer, 
a physician of Heilbronn in Germany. In certain 
observations connected with his medical practice it 
occurred to him that there must be a necessary 
equivalent between work and heat, a necessary 
numerical relation between them. “ The variations 
of the difference of colour of arterial and venous 
blood directed his attention to the theory of respira¬ 
tion. He soon saw in the respiration of animals the 
origin of their motive powers, and the comparison 
of animals to thermic machines afterwards suggested 
to him the important principle with which his name 
will remain for ever connected.” 

Next in order of publication of his results stands 
the name of Colding, a Danish engineer, who about 
the year 1843 presented a series of memoirs on the 
steam-engine to the Royal Society of Copenhagen, 
in which he put forth views almost identical with 
those of Mayer. 

Last in publication order, but foremost in the 
importance of his experimental treatment of the 
subject, was our own countryman, Dr Joule of 
Manchester. “ Entirely independent of Mayer, with 
his mind firmly fixed upon a principle, and undis¬ 
mayed by the coolness with which his first labours 
appear to have been received, he persisted for years 


26 GENERATION OF COSMICAL HEAT [chap. 

in his attempts to prove the invariability of the 
relation which subsists between heat and ordinary 
mechanical power. ” (We are quoting from Professor 
Tyndall’s valuable work on Heat considered as a 
Mode of Motion.) “He placed water in a suitable 
vessel, agitated the water by paddles, and deter¬ 
mined both the amount of heat developed by the 
stirring of the liquid and the amount of labour 
expended in its production. He did the same with 
mercury and sperm oil. He also caused discs of 
cast-iron to rub against each other, and measured 
the heat produced by their friction, and the force 
expended in overcoming it. He urged water through 
capillary tubes, and determined the amount of heat 
generated by the friction of the liquid against the 
sides of the tubes. And the results of his experi¬ 
ments leave no shadow of doubt upon the mind that, 
under all circumstances, the quantity of heat gener¬ 
ated by the same amount of force is fixed and in¬ 
variable. A given amount of force, in causing the 
iron discs to rotate against each other, produced 
precisely the same amount of heat as when it was 
applied to agitate water, mercury, or sperm oil. . . . 
The absolute amount of heat generated by the same 
expenditure of power, was in all cases the same. 

“ In this way it was found that the quantity of 


ii ] THE MECHANICAL EQUIVALENT OF HEAT 27 

heat which would raise one pound of water one 
degree Fahrenheit in temperature, is exactly equal 
to what would be generated if a pound weight, 
after having fallen through a height of 772 feet, had 
its moving force destroyed by collision with the 
earth. Conversely, the amount of heat necessary 
to raise a pound of water one degree in temperature, 
would, if all applied mechanically, be competent to 
raise a pound weight 772 feet high, or it would 
raise 772 pounds one foot high. The term ‘foot¬ 
pounds’ has been introduced to express in a con¬ 
venient way the lifting of one pound to the height 
of a foot. Thus the quantity of heat necessary to 
raise the temperature of a pound of water one degree 
Fahrenheit being taken as a standard, 772 foot¬ 
pounds constitute what is called the mechanical 
equivalent of heat.” 

By a process entirely different, and by an inde¬ 
pendent course of reasoning, Mayer had, a few 
months previous to Joule, determined this equiva¬ 
lent to be 771*4 foot-pounds. Such a remarkable 
coincidence arrived at by pursuing different routes 
gives this value a strong claim to accuracy, and 
raises the Mechanical Theory of Heat to the dignity 
of an exact science, and its enunciators to the fore¬ 
most place in the ranks of physical philosophers. 


28 


GENERATION OF COSMICAL HEAT [chap. 


In linking together the labours of the two 
remarkable men above alluded to, Professor Tyn¬ 
dall remarks, that “ Mayer’s labours have in some 
measure the stamp of profound intuition, which rose 
however to the energy of undoubting conviction in 
the author’s mind. Joule’s labours, on the contrary, 
are an experimental demonstration. Mayer thought 
his theory out, and rose to its grandest applications. 
Joule worked his theory out, and gave it the solidity 
of natural truth. True to the speculative instinet 
of his country, Mayer drew large and mighty 
conclusions from slender premises; while the 
Englishman aimed above all things at the firm 
establishment of facts. ... To each belongs a 
reputation which will not quickly fade, for the 
share he has had, not only in establishing the 
dynamical theory of heat, but also in leading the 
way towards a right appreciation of the general 
energies of the universe.” 

But from these generalities we must pass to the 
application of the mechanical theory of heat to our 
special subject. We have learnt that every form of 
motion is convertible into heat. We know that the 
falling meteor or shooting star, whose motion is im¬ 
peded by friction against the earth’s atmosphere, is 
heated thereby to a temperature of incandescence. 


n.] THE FORMATION OF WORLDS 29 

Let ns then suppose that myriads of such cosmical 
particles come into collision from the effect of their 
mutual attraction, or that the component atoms of 
a vast nebulous mass violently converged under the 
like influence. What would follow ? Obviously the 
generation of an intense heat by the arrest of con¬ 
verging motion, such a heat as would result in the 
fusion of the whole into one mass. Mayer, in one of 
his most remarkable papers (“ Celestial Dynamics ”) 
remarks that the “ Newtonian theory of gravitation, 
whilst it enables us to determine from its present 
form the earth’s state of aggregation in ages past, 
at the same time points out to us a source of heat 
powerful enough to produce such a state of aggrega¬ 
tion—powerful enough to melt worlds: it teaches 
us to consider the molten state of a planet as the 
result of the mechanical union of cosmical masses, 
and to derive the radiation of the sun and the 
heat in the bowels of the earth from a common 
origin.” 

And the same laws that governed the formation 
of the earth, governed also the formation of the 
moon: the variations of Nature’s operations are 
quantitative only and not qualitative. The Divine 
Will that made the earth made the moon also, and 
the means and mode of working were the same for 


30 


GENERATION OF COSMICAL HEAT [chap. 


both. The geological phenomena of the earth afford 
unmistakable evidence of its original fluid or molten 
condition, and the appearance of the moon is as 
unmistakably that of a body once in an igneous 
or molten state. The enigma of the earth’s primary 
formation is solved by the application of the dynami¬ 
cal theory of heat. By this theory the generation 
of cosmical heat is removed from the quicksands of 
conjecture and established upon the firm ground of 
direct calculation : for the absolute amount of heat 
generated by the collision of a given amount of 
matter is (of course, with some little uncertainty) 
deducible from a mathematical formula. Mayer 
has computed the amount of heat that the matter 
of the earth would have generated, if it had been 
formed originally of only two parts drawn into col¬ 
lision by their mutual attraction, and has found that 
it would be from 0 to 32,000 or 47,000 # Centigrade 
degrees, according as one part was infinitely small 
as compared with the other, or as the two parts 
were of equal size. Professor Helmholtz, another 
labourer in the same field of science, has computed 
the amount of heat generated by the condensation of 
the whole of the matter composing the solar system : 
this he finds would be equivalent to the heat that 

* The melting temperature of iron is 1500° Centigrade, 


ii ] THE MOON IN A STATE OF FUSION 31 

would be required to raise the temperature of a 
mass of water equal to the sum of the masses of all 
the bodies of the system to 28,000,000 (twenty-eight 
million) degrees of the Centigrade scale. 

These examples afford abundant evidence of 
sufficient heat having been generated by the aggre¬ 
gation of the matter of the moon to reduce it to a 
state of fusion, and so to produce, from a nebulous 
chaos of diffused cosmical matter, a molten body of 
definite outline and size. 

It is requisite here to remark that fusion does 
not necessarily imply combustion. It has been 
frequently asked, How can a volcanic theory of 
the lunar phenomena be upheld consistently with 
the condition that it possesses no atmosphere to 
support fire ? To this we would reply that to 
produce a state of incandescence or a molten con¬ 
dition it is not necessary that the body be sur¬ 
rounded by an atmosphere. The intensely rapid 
motion of the particles of matter of bodies, which 
the dynamical theory shows to be the origin of the 
molten state, exists quite independently of such 
external matter as an atmosphere. The complex 
mixture of gases and vapours which we term “air,” 
has nothing whatever to do with the fusion of sub¬ 
stances, whatever it may have to do with their 


32 


GENERATION OF COSMICAL HEAT [chap. 


combustion. Combustion is a chemical pheno¬ 
menon, due to the combination of the oxygen of 
that air with the heated particles of the combustible 
matter : oxygen is the sole supporter of combustion, 
and hence combustion is to be regarded rather as 
a phenomenon of oxygen than as a phenomenon of 
the matter with which that oxygen combines. The 
greatest intensity of heat may exist without oxygen, 
and consequently without combustion. In support 
of this argument it will be sufficient to adduce, 
upon the authority of Dr Tyndall, the fact that a 
platinum wire can be raised to a luminous tempera¬ 
ture and actually fused in a perfect vacuum. 

But while the mass of condensing cosmical 
matter was thus accumulating and forming the 
globe of the moon, the heat consequent upon the 
aggregation of its particles was suffering some 
diminution from the effect of radiation. So long 
as the radiated heat lost fell short of the dynamical 
heat generated, no effect of cooling would be 
manifest; but when the vis viva of the condensing 
matter was all converted into its equivalent of heat, 
or when the accession of heat fell short of that 
radiated, a necessary cooling must ensue, and this 
cooling would be accompanied by a solidification 
of that part of the mass which was most free to 


ii] THE “YEAR ONE” OF THE MOON’S LIFE 33 

radiate its heat into surrounding space: that part 
would obviously be the outer surface. 

With the solidification of this external crust 
began the “year one” of selenological history. 

The phenomena attendant upon the cooling of 
the mass we will consider in the next chapter. 


c 


CHAPTER III 

THE SUBSEQUENT COOLING OF THE IGNEOUS BODY 

In the foregoing chapters we have endeavoured to 
show, by the light of modern science, first, how 
diffused cosmical matter was probably condensed 
into a planetary mass by the mutual gravitation of 
its particles, and secondly, how, the after destruc¬ 
tion of the gravitative force, by the collision of the 
converging particles of matter, resulted in the 
generation of such sufficient heat as to reduce the 
whole mass to a molten condition. Our present 
task is to consider the subsequent cooling of the 
mass, and the phenomena attendant upon or result¬ 
ing therefrom. This brief chapter is important to 
our subject, as we shall have frequent occasion to 
refer to the leading principle we shall endeavour to 
illustrate in it, in subsequently treating of the 
causes to which the special selenological features 
are to be attributed. 

34 


chap, in.] THE COOLING OF THE MOON 


35 


First, then, as regards the cooling of the igneous 
mass that constituted the moon at the inconceivably 
remote period when possibly that body was really 
“a lesser light” shining with a luminosity of its 
own, due to its then incandescent state, and not 
simply a reflector, as it is now, of light which it 
receives from the sun. If we could conceive it 
possible that the igneous mass in the act of cooling 
parted with its heat from the central part first and 
so began to solidify from its centre, or if it had 
been possible for the mass to have cooled uniformly 
and simultaneously throughout its whole depth, or 
that each substratum had cooled before its super¬ 
stratum, we should have had a moon whose surface 
would have been smooth and without any such 
remarkable asperities and excrescences as are now 
presented to our view. But these suppositions are 
inadmissible : on the contrary we are compelled to 
consider that the portion of the igneous or molten 
body that first cooled was its exterior surface, 
which, radiating its heat into surrounding space, 
became solid and comparatively cool while the 
interior retained its hot and molten condition. So 
that at this early stage of the moon’s history it 
existed in the form of a solid shell inclosing a 
molten interior. 


36 COOLING OF THE IGNEOUS BODY [chap. 

Now at this period of its formation, the moon’s 
mass, partly cooled and solidified and partly molten, 
would be subject to the influence of two powerful 
molecular forces: the first of these would consist 
in the diminution of bulk or contraction of volume 
which accompanies the cooling of solidified masses 
of previously molten substances; the second would 
arise from a phenomenon which we may here 
observe is by no means so generally known as from 
its importance it deserves to be: and as we shall 
have frequent occasion to refer to it as one of the 
chief agencies in producing the peculiar structural 
characteristics of the moon’s surface, it may be 
well here to give a few examples of its action, that 
our reference to it hereafter may be more clearly 
understood. 

The broad general principle of the phenomenon 
here referred to is this :—that fusible substances are 
(with a few exceptions) specifically heavier while in 
their molten condition than in the solidified state, 
or in other words that molten matter occupies less 
space, weight for weight, than the same matter 
after it has passed from the melted to the solid 
condition. It follows as an obvious corollary that 
such substances contract in bulk in fusing or melt¬ 
ing, and expand in becoming solid. It is this 


hi] THE DENSITY OF MOLTEN SUBSTANCES 37 

expansion upon solidification that now concerns 
us. 

Water, as is well known, increases in density 
as it cools, till it reaches the temperature of 39° 
Fahrenheit, after which, upon a further decrease of 
temperature, its density begins to decrease, or in 
other words its bulk expands, and hence the well- 
known fact of ice floating in water, and the incon¬ 
venient fact of water-pipes bursting in a frost. 
This action in water is of the utmost importance in 
the grand economy of nature, and it has been 
accepted as a marvellous exception to the general 
law of substances increasing in density (or shrinking) 
as they decrease in temperature. Water is, how¬ 
ever, by no means the exceptional substance that 
it has been so generally considered. It is a fact 
perfectly familiar to ironfounders, that when a 
mass of solid cast-iron is dropped into a pot of 
molten iron of identical quality, the solid is found 
to float persistently upon the molten metal—so 
persistently that when it is intentionally thrust to 
the bottom of the pot, it rises again the moment the 
submerging agency is withdrawn. As regards the 
amount of buoyancy we believe it may be stated in 
round numbers to be at least two or three per 
cent. It has been suggested by some who are 


38 COOLING OF THE IGNEOUS BODY [chap. 

familiar with this phenomenon that the solid mass 
may be kept up by a spurious buoyancy imparted 
to it by a film of adhering air, or that surface im¬ 
purities upon the solid metal may tend to reduce 
the specific gravity of the mass and thereby prevent 
its sinking, and that the fact of flotation is not 
absolutely a proof of greater specific lightness. 
But in controversion of the suggestions, we can 
state as the result of experiment that pieces of 
cast-iron which have had their surface roughness 
entirely removed, leaving the bright metal exposed, 
still float on the molten metal, and further that 
when, under the influence of the great heat of the 
molten mass, the solid is gradually melted away, 
and consequently any possible surface impurities or 
adhering air must necessarily have been removed, 
the remaining portion continues to float to the last. 
The inevitable inference from this is that in the case 
of cast-iron the solid is specifically lighter than the 
molten, and, therefore, that in passing from the 
molten to the solid condition this substance under¬ 
goes expansion in bulk. 

We are able to offer a confirmation of this 
inference in the case of cast-iron by a remarkable 
phenomenon well known to ironfounders, but of 
which we have never met with special notice. 


III.] 


A PHENOMENON 


39 


When a ladle or pot of molten iron is drawn from 
the melting furnace and allowed to stand at rest, 
the surface presents a most remarkable and sug¬ 
gestive appearance. Instead of remaining calm 
and smooth it is a scene of a lively commotion: 
the thin coat of scoria or molten oxide which forms 
on the otherwise bright surface of the metal is seen, 
as fast as it forms at the circumference of the pot, 
to be swept by active convergent currents towards 
the centre, where it accumulates in a patch. 
While this action is proceeding, the entire upper 
surface of the metal appears as if it were covered 
with animated vermicules of scoria, springing into 
existence at the circumference of the pot, and from 
thence rapidly streaming and wriggling themselves 
towards the centre. 

Our illustration (Fig. 1) is intended, so far as 
such means can do so, to convey some idea of this 
remarkable appearance at one instant of its con¬ 
tinued occurrence. To interpret our illustration 
rightly it is necessary to imagine this vermicular 
freckling to be constantly and rapidly streaming 
from all points of the periphery of the pot towards 
the centre, where, as we have said, it accumulates 
in the form of a floating island. We may observe 
that the motion is most rapid when the hot metal 


40 


COOLING OF THE IGNEOUS BODY (chap. 


is first put into the cool ladle : as the fluid metal 
parts with some of its heat and the ladle gets hot 
by absorbing it, this remarkable surface disturbance 
becomes less energetic. 

Now if we carefully consider this peculiar action 
and seek a cause for the phenomenon, we shall be 
led to the conclusion that it arises from the expan¬ 
sion of that portion of the molten mass which is in 
contact with or close proximity to the compara¬ 
tively cool sides of the ladle, which sides act as the 
chief agent in dispersing the heat of the melted 
metal. The motion of the scoriae betrays that of 
the fluid metal beneath, and careful observation 
will show that the motion in question is the result 
of an upward current of the metal around the cir¬ 
cumference of the ladle, as indicated by the arrows 
a, b, c in the accompanying sectional drawing of 
the ladle (Fig. 2). The upward current of the 
metal can actually be seen when specially looked 
for, at the rim of the pot, where it is deflected into 
the convergent horizontal direction and where it 
presents an elevatory appearance as shown in the 
figure. It is difficult to assign to this effect any 
other cause than that of an expansion and conse¬ 
quent reduction of the specific gravity of the fluid 
metal in contact with or in close proximity to the 



Fig. 2. 

[ To face page 40, 


















































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: 















































< § " ■ /- 

V- 


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m.] EXPANSIVE SOLIDIFICATION 41 

cooler sides of the pot, as, according to the generally 
entertained idea that contraction universally accom¬ 
panies cooling, it would be impossible for the 
cooler to float on the hotter metal, and the curious 
surface-currents above referred to would be in con¬ 
trary direction to that which they invariably take, 
i.e., they would diverge from the centre instead of 
converging to it. The external arrows in the figure 
represent the radiation of the heat from the outer 
sides of the pot, which is the chief cause of cooling. 

Turning from cast-iron to other metals we find 
further manifestations of this expansive solidifica¬ 
tion. Bismuth is a notable example. In his 
lectures on Heat, Dr Tyndall exhibited an experi¬ 
ment in which a stout iron bottle was filled with 
molten bismuth, and the stopper tightly closed. 
The whole was set aside to cool, and as the metal 
within approached consolidation the bottle was 
rent open by its expansion, just as would have 
been the case had the bottle been filled with water 
and exposed to freezing temperature. Mercury 
affords another example. Thermometers which 
have to be exposed to Arctic temperatures are 
generally filled with spirit instead of quicksilver, 
because the latter has been found to burst the 
bulbs when the cold reached the congealing point 


42 COOLING OF THE IGNEOUS BODY [chap. 

of the metal, the bursting being a consequence of 
the expansion which accompanies the act of con¬ 
gelation. Silver also expands in passing from the 
fluid to the solid state, for we are informed by a 
practical refiner that solid floats on molten silver as 
ice floats on water; it also, as likewise do gold and 
copper, exhibits surface converging currents in the 
melting-pot like those depicted above for molten 
iron. 

It may, however, be objected that metals are 
too distantly related to volcanic substances to 
justify inferences being drawn from their behaviour 
in explanation of volcanic phenomena. With a 
view therefore of testing the question at issue with 
a substance admitted as closely allied to volcanic 
material, we appealed to the furnace slag of iron¬ 
works. The following are extracts from the letters 
of an iron manufacturer of great experience'* to 
whom we referred the question :— 

“I beg to inform you that cold slag floats in 
molten slag in the same way cold iron floats in 
molten iron. 

* Mr T. Heunter, Manager of the Ironworks of James 
Murry, Esq., of Dalmellington, Ayrshire. Another authority 
(Mr Snelus, of the West Cumberland Iron Company), writes as 
follows : “ I had a hole dug on the ‘ cinder-fall/ and allowed the 
running slag to flow through it so as to form a tolerably large 


III.] 


EXPERIMENTS 


43 


“ I filled a box with hot molten slag run quickly 
from a blast furnace; the box was about feet 
square by 2 feet deep, and I dropped into the slag 
a piece of cold slag weighing 16 lbs., when it came 
to the top in a second. I pushed it down to the 
bottom several times and it always made its appear¬ 
ance at the top: indeed a small portion of it re¬ 
mained above the molten slag.” 

Here then we have a substance closely allied to 
volcanic material which manifests the expansile 
principle in question; but we may go still further 
and give evidence from the very fountain-head by 
instancing what appears to be a most cogent 
example of its operation which we observed on the 
occasion of a visit to the crater of Vesuvius in 1865 
while a modified eruption was in progress. On 
this occasion we observed white-hot lava streaming 
down from apertures in the sides of a central cone 
within the crater and forming a lake of molten lava 
on the plateau or bottom of the crater; on the 

pool and yet keep fluid. Any crust that formed was skimmed 
off. A portion of the same slag was cooled, and the solid lump 
thrown into the pool. It floated just at the surface.” Mr 
Snelus adds, by the way, that he tried “ Bessemer-Pig ” in the 
same way, and that the solid pig sunk in the molten for a 
minute and then rose and floated just at the surface, with about 
one-twentieth of its bulk above the level of the fluid. 


44 COOLING OF THE IGNEOUS BODY [chap. 

surface of this molten lake vast cakes of the same 
lava which had become solidified were floating, 
exactly in the same manner as ice floats in water. 
The solidified lava had cracked, and divided into 
cakes, in consequence of its contraction and also 
of the uprising of the accumulating fluid lava on 
which it floated, more and more space being thus 
afforded for it to separate, on account of the crater 
widening upwards, while through the joints or 
fissures the fluid lava could be seen beneath. But 
for the decrease in density and consequent expan¬ 
sion in volume which accompanied solidification, 
this floating of the solidified lava on the molten 
could not have occurred. Deference to Fig. 3, 
which represents a section of the crater of Vesuvius 
on the occasion above referred to, will perhaps 
assist the reader to a more clear idea of what we 
have endeavoured to describe, a a are the streams 
of white-hot lava issuing from openings in the sides 
of the central cone, and accumulating beneath the 
solidified crust b b in the lake of molten lava at 
c c; the solidified crust b b as it was floated up¬ 
wards dividing into separate cakes as represented 
in Fig. 4. (See also Plate I.) 

Let us now consider what would be the effect 
produced upon a spherical mass of molten matter 



[To face page 44 


Plate I. — The Crater of Vesuvius in 1864 






















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[To face page 44. 










































4 




III.] 


THE EXTERNAL SURFACE 


45 


in progress of cooling, first under the action of the 
above described expansion which precedes solidifi¬ 
cation, and then by the contraction which accom¬ 
panies the cooling of a solidified body. The first 
portion of such a mass to part with its heat being 
its external surface, this portion would expand, but 



there being no obstacle to resist the expansion 
there would be no other result than a temporary 
slight enlargement of the sphere. This external 
portion would on cooling form a solid shell encom¬ 
passing a more or less fluid molten nucleus, but as 
this interior has in its turn, on approaching the 
point of solidification, to expand also, and there 


46 


COOLING OF THE IGNEOUS BODY [chap. 


being, so to speak, no room for its expansion, by 
reason of its confinement within its solid casing, 
what would be the consequence ?—the shell would 
be rent or burst open, and a portion of the molten 
interior ejected with more or less violence accord¬ 
ing to circumstances, and many of the characteristic 
features of volcanic action would be thus pro¬ 
duced : the thickness of the outer shell, the size of 
the vent made by the expanding matter for its 
escape, and other conditions conspiring to modify 
the nature and extent of the eruption. Thus there 
would result vast floodings of the exterior surface 
of the shell by the so extruded molten matter, 
volcanoes, extruded mountains, and other mani¬ 
festations of eruptive phenomena. The sectional 
diagram (Fig. 5 ) will help to convey a clear idea 
of this action. Basing our reasoning on the prin¬ 
ciple we have thus enunciated, namely, that molten 
telluric matter expands on nearing the point of 
solidification, and which we have endeavoured to 
illustrate by reference to actual examples of its 
operation, we consider we are justified in assuming 
that such a course of volcanic phenomena has very 
probably occurred again and again upon the moon; 
that this expansion of volume which accompanies 
the solidification of molten matter furnishes a key 



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[To /ace page 46. 




































THE OUTER CRUST 


47 


in.] 

to the solution of the enigma of volcanic action; 
and that such theories as depend upon the agency 
of gases, vapour, or water are at all events un¬ 
tenable with regard to the moon, where no gases, 
vapour, or water, appear to exist. 

That an upheaving and ejective force has been 
in action with varying intensity beneath the whole 
of the lunar surface is manifest from the aspect 
of its structural details, and we are impressed with 
the conviction that the principle we have set forth, 
namely the paroxysms of expansion which succes¬ 
sively occurred as portions of its molten interior 
approached solidification, supply us with a rational 
cause to which such vast ejective and upheaving 
phenomena may be assigned. Many features of 
terrestrial geology likewise require such an expan¬ 
sive force whereby to explain them; we therefore 
venture to recommend this source and cause of 
ejective action to the careful consideration of 
geologists. 

When the molten substratum had burst its 
confines, ejected its superfluous matter, and pro¬ 
duced the resulting volcanic features, it would, 
after final solidification, resume the normal process 
of contraction upon cooling, and so retreat or 
shrink away from the external shell. Let us now 


48 COOLING OF THE IGNEOUS BODY [chap. 

consider what would be the result of this. Evi¬ 
dently the external shell or crust would become 
relatively too large to remain at all points in close 
contact with the subjacent matter. The conse¬ 
quence of too large a solid shell having to accom¬ 
modate itself to a shrunken body underneath, is 
that the skin, so to term the outer stratum of solid 
matter, becomes shrivelled up into alternate ridges 
and depressions, or wrinkles. In its attempt to 
crush down and follow the contracting substratum 
it would have to displace the superabundant or 
superfluous material of its former larger surface by 
thrusting it (by the action of tangential force) into 
undulating ridges as in Fig. 6, or broken elevated 
ridges as in Fig. 7, or overlappings of the outer 
crust as in Fig. 8, or ridges capped by more or less 
fluid molten matter extruded from beneath, as 
indicated in Fig. 9, a class of action which might 
occur contemporaneously with the elevation of the 
ridge or subsequently to its formation. 

A long-kept shrivelled apple affords an apt 
illustration of this wrinkle theory; another example 
may be observed in the human face and hand, 
when age has caused the flesh to shrink and so 
leave the comparatively unshrinking skin relatively 
too large as a covering for it. We illustrate both 



Plate II. — Back of Hand, to illustrate the origin of certain mountain 
ranges resulting from shrinkage of the interior. 


[To face page 48. 





V 












* 







* 








* 







-* 










4 

















* 



4 






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Plate III.—Shrivelled Apple, to illustrate the origin of certain ranges 
resulting from shrinkage of the interior of the globe. 


[To face page 48 








[To face page 48. 


f 


Fig. 



















hi] WRINKLES ON EARTH AND MOON 


49 


of these examples by actual photographs of the 
respective objects, which are reproduced on Plates 
II. and III. Whenever an outer covering has to 
accommodate and apply itself to an interior body 
that has become too small for it, wrinkles are in¬ 
evitably produced. The same action that shrivels 
the human skin into creases and wrinkles, has 
also shrivelled certain regions of the igneous crust 
of the earth. A map of a mountainous part of 
our globe affords abundant evidence of such a 
cause having been in action; such maps are 
pictures of wrinkles. Several parts of the lunar 
surface, as we shall by-and-by see, present us 
with the same appearances in a modified degree. 

To the few primary causes we have set forth 
in this chapter—to the alternate expansion and 
contraction of successive strata of the lunar sphere, 
when in a state of transition from an igneous and 
molten to a cooled and solidified condition, we 
believe we shall be able to refer well-nigh all the 
remarkable and characteristic features of the lunar 
surface which will come under our notice in the 
course of our survey. 


D 


CHAPTER IY 

THE FORM, MAGNITUDE, WEIGHT, AND DENSITY OF THE 
LUNAR GLOBE 

We have not hitherto had occasion to refer to what 
we may term the physical elements of the moon : by 
which we mean the various data concerning form, 
size, weight, density, etc. of that body, derived from 
observation and calculation. To this purpose, there¬ 
fore, we will now devote a few pages, confining our¬ 
selves to such matters as specially bear upon the 
requirements of our subject, omitting such as are 
irrelevant to our purpose, and touching but lightly 
upon such as are commonly known, or are explained 
in ordinary elementary treatises on astronomy. 

First, then, as regards the form of the moon. 
The form of the lunar disc, when fully illuminated, 
we perceive to be a perfect circle; that is to say, 
the measured diameters in all directions are equal; 
and we are therefore led to infer that the real form 

50 


CHAP. IV.] the moon an ellipsoid 


51 


of the moon is that of a perfect sphere. We know 
that the earth and the rest of the planets of our 
system are spheroidal, or more or less flattened at 
the poles, and we also know that this flattening is 
a consequence of axial rotation; the extent of the 
flattening, or the oblateness of the spheroid, depend¬ 
ing upon the speed of that rotation. But in the 
case of the moon the axial rotation is so slow that 
the flattening produced thereby, although it must 
exist, is so slight as to be imperceptible to our 
observation. We might therefore conclude that 
the moon is a perfectly spherical body, did not 
theory step in to show us that there is another 
cause by which its form is disturbed. Assuming 
the moon to have been once in a fluid state, it is 
demonstrable that the attraction of the earth would 
accumulate a mass of matter, like a tidal elevation, 
in the direction of a line joining the centres of the 
two bodies: and as a consequence, the real shape 
of the moon must be an ellipsoid, or somewhat egg- 
shaped body, the major axis of which is directed 
towards the earth. That some such phenomenon 
has obtained is evident from the coincidence of the 
times of orbital revolution and axial rotation of the 
lunar sphere. “ It would be against all probability,” 
says Laplace, “to suppose that these two motions 


52 FORM, MAGNITUDE, WEIGHT, ETC. [chap. 

had been at their origin perfectly equal ”; but it is 
sufficient that their primitive difference was but 
small, in which case the constant attraction by the 
earth of the protuberant part of the moon would 
establish the equality which at present exists. 

It is, however, sufficient for all purposes with 
which we are concerned to regard the moon as a 
sphere, and the next point to be considered is its 
size. To determine this, two data are necessary 
—its apparent or angular diameter, and its distance 
from the earth. The first of these is obtained by 
measuring the angle comprised between two lines 
directed from the eye to two opposite “limbs” or 
edges of the moon. If, for instance, we were to 
take a pair of compasses and, placing the joint at 
the eye, open out the legs till the two points appear 
to touch two opposite edges of the moon, the two 
legs would be inclined at an angle which would re¬ 
present the diameter of the moon, and this angle we 
could measure by applying a divided arc or pro¬ 
tractor to the compasses. In practice this measure¬ 
ment is made by means of telescopes attached to 
accurately divided circles; the difference between 
the readings of the circle when the telescope is 
directed to opposite limbs of the moon giving its 
angular diameter at the time of the observation. 


IV.] 


MEAN DIAMETER OF THE MOON 


53 


But from the fact that the orbit of the moon is an 
ellipse, it is evident that she is at some times much 
nearer to us than at others, and, as a consequence, 
her apparent magnitude is variable; there is also a 
slight variation depending upon the altitude of the 
moon at the time of the measurement; the mean 
diameter, however, or the diameter at mean distance 
from the centre of the earth has, from long course 
of observation, been found to be 31' 9". 

To convert this apparent angular diameter into 
real linear measurement, it is necessary to know 
either the distance of the moon from the earth, or 
in astronomical language as leading to a knowledge 
of that distance, what is the amount of the moon’s 
‘parallax. Parallax generally, is an apparent change 
of position of an object arising from change of the 
point of view. The parallax of a heavenly body is 
the angle which the earth would subtend if it were 
seen from that body. Supposing an observer on 
the moon could measure the earth’s angular dia¬ 
meter, just as we measure that of the moon, his 
measurement would represent what is called the 
parallax of the moon. But we cannot go to the 
moon to make such a measurement; nevertheless 
there is a simple method, explained in most treatises 
on astronomy, which consists in observing the moon 


54 FORM, MAGNITUDE, WEIGHT, ETC. [chap. 

from stations on the earth widely separated, and by 
which we can obtain a precisely similar result. 
Without detailing the process, it is sufficient for us 
to know that the angle which would be subtended 
by the earth if seen from the moon, or the moon’s 
parallax, is according to the latest determination, 
equal to 1° 54' 5". This value, however, varies con¬ 
siderably with the variations of distance due to the 
elliptic orbit of the moon : the number we have 
given represents the mean parallax, or the parallax 
at mean distance. 

But we have to turn these angular measure¬ 
ments into miles. To effect this we have only to 
work a simple rule-of-three sum. It will easily be 
understood that, as the angular diameter of the 
earth seen from the moon is to the angular diameter 
of the moon seen from the earth, so is the diameter 
of the earth in miles to the diameter of the moon 
in miles. The diameter of the earth we know to 
be 7912 miles: putting this therefore in its proper 
place in the proportion sum, and duly working it 
out by the schoolboy’s rule, we get :— 

MILES. MILES. 

1 ‘ 54 '. 5 " : 31 '. 9 " :: 7912 : 2160 

And 2160 miles is therefore the diameter of the 
lunar globe. 










[To face page 55. 


Fig. 10 




















































































































































IV.] 


AREA AND VOLUME 


55 


Knowing the diameter, we can easily obtain the 
other elements of magnitude. According to the 
well-known relation of the diameter of a sphere to 
its area, we find the area of the moon to be 
14,657,000 square miles: or half that number, 
7,328,500 miles, as the area of the hemisphere at 
any one time presented to our view. And simi¬ 
larly, from the relation of the solidity of a sphere to 
its diameter, we find the solid contents of the moon 
to be 5276 millions of cubic miles of matter. 

Comparing these data with corresponding 
dimensions of the earth, we find that the diameter 
of the moon is ; the area ; and the volume 
igKfc, of the respective elements of the earth. Those 
who prefer a graphical to a numerical comparsion, 
may judge of the sizes of the two bodies by the 
accompanying illustration (Fig. 10). To gain an 
idea of their distance from each other it is neces¬ 
sary to suppose the two discs in the diagram to be 
five feet apart; the real distance of the moon from 
the earth being about 238,790 miles at its mean 
position. 

Next, we come to what is technically termed 
the mass, but what in common language we may call 
the weight of the moon. It is important to know 
this, because the weight of a body taken in connec- 


56 FORM, MAGNITUDE, WEIGHT, ETC. [chap. 

tion with its size furnishes us with a knowledge of 
its density, or the specific gravity of the material of 
which it is composed. But it is not quite so easy 
to determine the mass as the dimensions of the 
moon : to measure it, we have seen is easy enough; 
to weigh it is a comparatively difficult matter. To 
solve the problem we have to appeal to Newton’s 
law of universal gravitation. This law teaches us 
that every particle of matter in the universe attracts 
every other particle with a force which is directly 
proportional to the mass , and inversely proportional 
to the square of the distance of the attracting par¬ 
ticles. There are several methods by which this 
law is applied to the measurement of the mass of 
the moon. One of the simplest is by the agency of 
the Tides. We know that the moon, attracting the 
waters, produces a certain amount of elevation of 
the aqueous covering of the earth; and we know 
that the sun produces also a like elevation, but to a 
much smaller extent, by reason of its much greater 
distance. Now measuring accurately the heights of 
the solar and lunar tides, and making allowance for 
the difference of distance of the sun and moon from 
the earth, we can compare directly the effect that is 
due to the sun with the effect that is due to the 
moon : and since the masses of the two bodies are 


IV,] 


THE MASS OF THE MOON 


57 


just in proportion to the effects they produce, it is 
evident that we have a comparison between the 
mass of the sun and that of the moon; and knowing 
what is the sun’s mass we can, by simple proportion, 
find that of the moon. Another method is as 
follows :—The moon is retained in her orbital path 
by the attraction of the earth; if it were not for this 
attraction she would fly off from her course in a 
tangential line. She has thus a constant tendency 
to quit her orbit, which the earth’s attraction as 
constantly overcomes. It is evident from this that 
the earth pulls the moon towards itself by a definite 
amount in every second of time. But while the 
earth is pulling the moon, the moon is also pulling 
the earth: they are pulling each other together; 
and moreover each is exerting a pull which is 'pro¬ 
portional to its mass. Knowing, then, the mass of 
the earth, which we do with considerable accuracy, 
we can find what share of the whole pulling force 
is due to it, the residue being the moon’s share: 
the proportion which this residue bears to the earth’s 
share gives us the proportion of the moon’s mass to 
that of the earth, and hence the mass of the moon. 

There are yet two other methods : one depending 
upon the phenomena of nutation, or the attraction 
of the sun and moon upon the protuberant matter 


58 FORM, MAGNITUDE, WEIGHT, ETC. [chap. 

of the terrestrial spheroid; and the other upon a 
displacement of the centre of gravity of the earth 
and moon, which shows itself in observations of the 
sun. By each and all of these methods has the lunar 
mass been at various times determined, and it has 
been found, as the latest and best accepted value, 
that the mass of the moon is one-eightieth that of 
the earth. 

From the known diameter of the earth we ascer¬ 
tain that its volume is 259,360 millions of cubic 
miles : and from the various experiments that have 
been made to determine the mean density of the 
earth, it has been found that that mean density 
is about 5|- times that of water; that is to say, the 
earth weighs 5J times heavier than would a sphere 
of water of equal size. Now a cubic foot of water 
weighs 62*3211 pounds, and from this we can find 
by simple multiplication what is the weight of a 
cubic mile of water, and, similarly, what would be 
the weight of 259*360 cubic miles of water, and the 
last result multiplied by 5J will give the weight of 
the earth in tons: The calculation, although ex¬ 
tremely simple, involves a confusing heap of figures; 
but the result, which is all that concerns us, is, that 
the weight of the earth is 5842 trillions of tons : and 
since, as we have above stated, the mass of the earth 


IV.] DENSITY OF THE LUNAR MATTER 


59 


is 80 times that of the moon, it follows that the 
weight of the moon is 73 trillions of tons. 

The cubical contents of a body compared with its 
weight gives us its density. In the moon we have 
5276 millions of cubic miles of matter, the total 
weight of which is 73 trillions of tons. Now, 5276 
millions of cubic miles of water would weigh about 
21^ trillions of tons; and as this number is to 73 as 
1 is to 3*4, it is clear that the density of the lunar 
matter is 3*4 greater than water : and inasmuch as 
the earth is 5|- times denser than water, we see that 
the moon is about 062 as dense as the earth, or that 
the material of the moon is lighter, bulk for bulk, 
than the mean material of the terraqueous globe in 
the proportion of 62 to 100, or, nearly 6 to 10. This 
specific gravity of the lunar material (3*4) we may 
remark is about the same as that of flint glass or 
the diamond: and curiously enough it nearly co¬ 
incides with that of the aerolites that have from 
time to time fallen to the earth; hence support has 
been claimed for the theory that these bodies were 
originally fragments of lunar matter, probably 
ejected at some time from the lunar volcanoes with 
such force as to propel them so far within the sphere 
of the earth’s attraction that they have ultimately 
been drawn to its surface. 


60 FORM, MAGNITUDE, WEIGHT, ETC. [chap. 

Reverting, now, to the mass of the moon: we 
must bear in mind that the mass or weight of a 
planetary body determines the weight of all objects 
on its surface. What we call a pound on the earth, 
would not be a pound on the moon; for the follow¬ 
ing reason :—When we say that such and such an 
object weighs so much, we really mean that it is 
attracted towards the earth with a certain force 
depending upon its own weight. This attraction 
we call gravity; and the falling of a weight to the 
earth is an example of the action of the law of uni¬ 
versal gravitation. The earth and the weight fall 
together-—or are held together if the weight is in 
contact with the earth—with a force which depends 
directly upon the mass of the two, and upon the 
distance between them. Newton proved that the 
attraction of a sphere upon external objects is pre¬ 
cisely as if the whole of its matter were contained 
at its centre. So that the attractive force of the 
earth upon a ton weight at its surface is the attrac¬ 
tion which 5842 trillions of tons exert upon one ton 
situated 3956 miles (the radius of the earth) distant. 
If the weight of the earth were only half the above 
quantity, it is clear that the attraction would be 
only half what it is; and hence the ton weight, 
being pulled by only half the force, would only be 


IV.] FORCE OF GRAVITY UPON THE MOON 61 


equal to half a ton ; that is to say, only half as much 
muscular force (or any other force but gravity) would 
be required to lift it. It is plain, therefore, that 
what weighs a pound on the earth could not weigh 
a pound on the moon, which is only w of the weight 
of the earth. What, then, is the relation between 
a pound on the earth and the same mass of matter 
on the moon ? It would seem, since the moon’s 
mass is io of the earth, that the pound transported 
to the moon ought to weigh the eightieth part of a 
pound there; and so it would if the distance from 
the centre of the moon to its surface were the same 
as the distance of the centre of the earth from its 
surface. But the radius of the moon is only ^ that 
of the earth ; and the force of gravity varies inversely 
as the square of the distance between the centres of 
the gravitating masses. So that the attraction by 
the moon of a body at its surface, as compared with 
that of the earth, is sV divided by the square of 3^5 : 
and this, worked out, is equal to J. The force of 
gravity upon the moon is, therefore, \ of that on the 
earth ; and hence a pound upon the earth would be 
little more than 2 J ounces on the moon; and it 
follows as a consequence that any force, such as 
muscular exertion, or the energy of chemical, plu- 
tonic or explosive forces, would be six times more 


62 FORM, MAGNITUDE, WEIGHT, ETC. [chap. 

effective upon the moon than upon the earth. A 
man who could jump six feet from the earth, could 
with the same muscular effort jump thirty-six feet 
from the moon; the explosive energy that would 
project a body a mile above the earth would project 
a like body six miles above the surface of the 
moon. 

It is the practice, in elementary and popular 
treatises on astronomy, to state merely the numeri¬ 
cal results in giving data such as those embodied 
in the foregoing pages; and uninitiated readers, not 
knowing the means by which the figures are arrived 
at, are sometimes disposed to regard them with a 
certain amount of doubt or uncertainty. On this 
account we have thought it advisable to give, in 
as brief and concise a form as possible, the various 
steps by which these seemingly unattainable results 
are obtained. 

The data explained in the foregoing text are 
here collected to facilitate reference. 

Diameter of Moon 2160 miles - ——that of earth. 

3’665 

Area - - - 14,657,000 sq. miles - — „ 

Area of the visible'! ,7000 kaa -i 
, . u 17,328,500 sq. miles 

hemisphere -J n 

Solid contents 5276 millions of cub. miles —A— 



IV.] 

FACTS AND FIGURES 

63 

Mass - 

- 73 trillions of tons - 

80 

that of earth. 

Density 

- 3 39 (water * 1) - - 0*62 

)) >) 


6 

- 238,790 miles. 


Force of gravity at surface - 

Mean distance from earth 


CHAPTEB Y 

ON THE EXISTENCE OR NON-EXISTENCE OF A 
LUNAR ATMOSPHERE 

At the close of the preceding chapter we stated 
that any force acting in opposition to that of 
gravity would be six times more effective on the 
moon than on the earth. But, in fact, it would in 
many cases be still more so; at all events, so far 
as projectile forces are concerned; for the reason 
that “the powerful coercer of projectile range,” as 
the earth’s atmosphere has been termed, has no 
counterpart, or at most a very disproportionate 
one, upon the moon. 

The existence of an atmosphere surrounding 
the moon has been the subject of considerable 
controversy, and a great deal of evidence on both 
sides of the question has been offered from time 
to time, and is to be found scattered through the 
records of various classes of observations. Some 

64 


CHAP. V.] 


CLOUDS 


65 


of the more important items of this evidence it is 
our purpose to set forth in the course of the 
present chapter. 

With the phenomena of the terrestrial atmos¬ 
phere, with the effects that are attributable to 
it, we are all well familiar, and our best course 
therefore is to examine, as far as we are able, 
whether counterparts of any of these effects are 
manifested upon the moon. For instance, the 
clouds that are generated in and float through our 
air would, to an observer on the moon, appear as 
ever-changing bright or dusky spots, obliterating 
certain of the permanent details of the earth’s 
surface, and probably skirting the terrestrial disc, 
like the changing belts we perceive on the planet 
Jupiter, or diversifying its features with less regu¬ 
larity, after the manner exhibited by the planet 
Mars. If such clouds existed on the moon it is 
evident that the details of its surface must be, 
from time to time, similarly obscured; but no 
trace of such obscuration has ever been detected. 
When the moon is observed with high telescopic 
powers, all its details come out sharp and clear, 
without the least appearance of change or the 
slightest symptoms of cloudiness other than the 
occasional want of general definition, which may 


66 


ON THE LUNAR ATMOSPHERE [chap. 


be proved to be the result of unsteadiness or want 
of homogeneity in our own atmosphere; for we 
must tell the uninitiated that nights of pure, good 
definition, such as give the astronomer opportunity 
of examining with high powers the minute details 
of planetary features, are very few and far between. 
Out of the three hundred and sixty-five nights of a 
year there are probably not a dozen that an 
astronomer can call really fine: usually, even on 
nights that are to all common appearance superbly 
brilliant, some strata of air of different densities or 
temperatures, or in rapid motion, intervene between 
the observer and the object of his observation, and 
through these, owing to the ever-changing refrac¬ 
tions which the rays of light coming from the 
object suffer in their course, observation of the 
delicate markings of a planet is impossible: all is 
blurred and confused, and nothing but bolder 
features can be recognised. It has in consequence 
sometimes happened that a slight indistinctness of 
some minute detail of the moon has been attributed 
to clouds or mists at the lunar surface, whereas the 
real cause has been only a bad condition of our 
own atmosphere. It may be confidently asserted 
that when all indistinctness due to terrestrial 
causes is taken account of or eliminated, there 


THE CORONA 


v.] 


67 


remain no traces whatever of any clouds or mists 
upon the surface of the moon. 

This is but one proof against the existence of 
a lunar atmosphere, and, it may be argued, not a 
very conclusive one; because there may still be an 
atmosphere, though it be not sufficiently aqueous 
to condense into clouds and not sufficiently dense 
to obscure the lunar details. The probable exist¬ 
ence of an atmosphere of such a character used to 
be inferred from a phenomenon seen during total 
eclipses of the sun. On these occasions the black 
body of the moon is invariably surrounded by a 
luminous halo, or glory, to which the name 
“corona” has been applied; and, further, besides 
this corona, apparently floating in it and sometimes 
seemingly attached to the black edge of the moon, 
are seen masses of cloud-like matter of a bright red 
colour, which, from the form in which they were 
first seen and from their flame-like tinge, have 
become universally known as the “red flames.” It 
used to be said that this corona could only be the 
consequence of a lunar atmosphere lit up as it were 
by the sun’s rays shining through it, after the 
manner of a sunbeam lighting up the atmosphere 
of a dusty chamber; and the red flames were held 
by those who first observed them to be clouds of 


68 ON THE LUNAIi ATMOSPHERE [chap. 

denser matter floating in the said atmosphere, and 
refracting the red rays of solar light as our own 
clouds are seen to do at sunrise and sunset. But 
the evidence obtained, both by simple telescopic 
observation and by the spectroscope, from recent 
extensively observed eclipses of the sun has set this 
question quite at rest; for it has been settled finally 
and indisputably that both the above appearances 
pertain to the sun, and have nothing whatever to 
do with the moon. 

The occurrence of a solar eclipse offers other 
means in addition to the foregoing whereby a lunar 
atmosphere would be detected. We know that 
all gases and vapours absorb some portion of any 
light which may shine through them. If then 
our satellite had an atmosphere, its black nucleus 
when seen projected against the bright sun in an 
eclipse would be surrounded by a sort of penum¬ 
bra, or zone of shadow, in contact with its edge, 
somewhat like that we have shown in an exag¬ 
gerated degree in Figure 11, and the passage of this 
penumbra over solar spots and other features of 
the solar photosphere would to some extent ob¬ 
scure the more minute details of such features. 
No such dusky band has however been at any time 
observed. On the contrary, a band somewhat 





Fig. 11. 



Fig. 12. 


[To face page 68. 
















































t 



* 


























v.] 


VARIOUS EVIDENCE 


69 


brighter than the general surface of the sun has 
frequently been seen in contact with the black 
edge of the moon: this in its turn was held to 
indicate an atmosphere about the moon; but 
Sir George Airy has shown that a lunar atmos¬ 
phere, if it really did exist, could not produce 
such an appearance, and that the cause of it must 
be sought in other directions. If this effect were 
really due to the passage of the solar rays through 
a lunar atmosphere a similar effect ought to be 
produced by the passage of the sun’s rays through 
the terrestrial atmosphere: and we might hence 
expect to see the shadow of the earth projected on 
the moon during a lunar eclipse surrounded by a 
sort of bright zone or halo: we need hardly say 
such an appearance has never manifested itself. 
Similarly as we stated that the delicate details of 
solar spots would be obscured by a lunar atmos¬ 
phere, small stars passing behind the moon would 
suffer some diminution in brightness as they ap¬ 
proached apparent contact with the moon’s edge: 
this fading has been watched for on many occa¬ 
sions, and in a few cases such an appearance has 
been suspected, but in by far the majority of 
instances nothing like a diminution of brightness 
or change of colour of the stars has been seen; 


70 ON THE LUNAR ATMOSPHERE [chap. 

stars of the smallest magnitude visible under such 
circumstances retain their feeble lustre unimpaired 
up to the moment of their disappearance behind 
the moon’s limb. 

Again, in a solar eclipse, even if there were an 
atmosphere about the moon not sufficiently dense 
to form a hazy outline or impair the distinctness 
of the details of a solar spot, it would still mani¬ 
fest its existence in another way. As the moon 
advances upon the sun’s disc the latter assumes, of 
course, a crescent form. Now if air or vapour 
enveloped the moon, the exceedingly delicate cusps 
of this crescent would be distorted or turned out of 
shape. Instead of remaining symmetrical, like the 
lower one in Figure 12, they would be bent or 
deformed after the manner we have shown in the 
upper one. The slightest symptom of a distortion 
like this could not fail to obtrude itself upon an 
observer’s eye; but in no instance has anything of 
the kind been seen. 

Reverting to the consequences of the terrestrial 
atmosphere: one of the most striking of these is 
the phenomenon of diffused daylight, which we 
need hardly remind the reader is produced by the 
scattering or diffusion of the sun’s rays among the 
minute particles of vapour composing or contained 


V.] 


DIFFUSED DAYLIGHT 


71 


in that atmosphere. Were it not for this reflexion 
and diffusion of the sun’s light, those parts of 
our earth not exposed to direct sunshine would 
be hidden in darkness, receiving no illumination 
beyond the feeble amount that might be reflected 
from proximate terrestrial objects actually illumin¬ 
ated by direct sunlight. Twilight is a consequence 
of this reflexion of light by the atmosphere when 
the sun is below the horizon. If, then, an atmos¬ 
phere enveloped the moon, we should see by 
diffused light those parts of the lunar details that 
are not receiving the direct solar beams; and 
before the sun rose and after it had set upon 
any region of the moon, that region would still 
be partially illuminated by a twilight. But, on the 
contrary, the shadowed portions of a lunar lands¬ 
cape are pitchy black, without a trace of diffused- 
liglit illumination, and the effects that a twilight 
would produce are entirely absent from the moon. 
Once, indeed, one observer, Schroeter, noticed 
something which he suspected was due to an effect 
of this kind: when the moon exhibited itself as a 
very slender crescent, he discovered a faint crepus¬ 
cular light, extending from each of the cusps along 
the circumference of the unenlightened part of the 
disc, and he inferred from estimates of the length 


72 


ON THE LUNAR ATMOSPHERE [chap. 


and breadth of the line of light that there was an 
atmosphere about the moon of 5376 feet in height. 
This is the only instance on record, we believe, of 
such an appearance being seen. 

Spectrum analysis would also betray the exist¬ 
ence of a lunar atmosphere. The solar rays falling 
on the moon are reflected from its surface to the 
earth. If, then, an atmosphere existed, it is plain 
that the solar rays must first pass through such 
atmosphere to reach the reflecting surface, and re¬ 
turning from thence, again pass through it on them 
way to the earth; so that they must in reality pass 
through virtually twice the thickness of any atmos¬ 
phere that may cover the moon. And if there be 
any such atmosphere, the spectrum formed by the 
moon’s light, that is, by the sun’s light reflected 
from the moon, would be modified in such a manner 
as to exhibit absorption-lines different from those 
found in the spectrum of the direct solar rays, just 
as the absorption-lines vary according as the sun’s 
rays have to pass through a thinner or a denser 
stratum of the terrestrial atmosphere. Guided by 
this reasoning, Drs Huggins and Miller made 
numerous observations upon the spectrum of the 
moon’s light, which are detailed in the Philo¬ 
sophical Transactions for the year 1864; and their 


V] SPECTRUM OF AN OCCULTING STAR 73 


result, quoting the words of the report, was “ that 
the spectrum analysis of the light reflected from the 
moon is wholly negative as to the existence of any 
considerable lunar atmosphere.” 

Upon another occasion, Dr Huggins made an 
analogous observation of the spectrum of a star at 
the moment of its occupation, which observation he 
records in the following words :—“ When an obser¬ 
vation is made of the spectrum of a star a little 
before, or at the moment of its occupation by the 
dark limb of the moon, several phenomena char¬ 
acteristic of the passage of the star’s light through 
an atmosphere might possibly present themselves 
to the observer. If a lunar atmosphere exist, 
which either by the substances of which it is 
composed, or by the vapours diffused through it, 
can exert a selective absorption upon the star’s 
light, this absorption would be indicated to us by 
the appearance in the spectrum of new dark lines 
immediately before the star is occultated by the 
moon. 

“ If finely divided matter, aqueous or otherwise, 
were present about the moon, the red rays of the 
star’s light would be enfeebled in a smaller degree 
than the rays of higher refrangibilities. 

“ If there be about the moon an atmosphere free 


74 ON THE LUNAR ATMOSPHERE [chap. 

from vapour, and possessing no absorptive power, 
but of some density, then the spectrum would not 
be extinguished by the moon’s limb at the same 
instant throughout its length. The violet and 
blue rays would lie behind the red rays. 

“ I carefully observed the disappearance of the 
spectrum of e Piscium at its occultation of 4tli Janu¬ 
ary 1865, for these phenomena; but no signs of a 
lunar atmosphere were detected.” 

But perhaps the strongest evidence of the non¬ 
existence of any appreciable lunar atmosphere is 
afforded by the non-refraction of the light of a star 
passing behind the edge of the lunar disc. Refrac¬ 
tion, we know, is a bending of the rays of light 
coming from any object, caused by their passage 
through strata of transparent matter of different 
densities; we have a familiar example in the appar¬ 
ent bending of a stick when half plunged into water. 
There is a simple schoolboy’s experiment which 
illustrates refraction in a very cogent manner, but 
which we should, from its very simplicity, hesitate 
to recall to the reader’s mind did it not very aptly 
represent the actual case we wish to exemplify. A 
coin is placed on the bottom of an empty basin, and 
the eye is brought into such a position that the coin 
is just hidden behind the basin’s rim. Water is 


V] REFRACTION AND ATMOSPHERE 75 

then poured into the basin and, without the eye 
being moved from its former place, as the depth of 
water increases, the coin is brought by degrees fully 
into view; the water refracting or turning out of 
their course the rays of light coming from the coin, 
and lifting them, as it were, over the edge of the 
basin. Now a perfectly similar phenomenon takes 
place at every sunrise and sunset on the earth. 
When the sun is really below the horizon, it is 
nevertheless still visible to us because it is brought 
up by the refraction of its light by the dense stratum 
of atmosphere through which the rays have to pass. 
The sun is, therefore, exactly represented by the 
coin at the bottom of the basin in the boy’s experi¬ 
ment, the atmosphere answers to the water, and the 
horizon to the rim or edge of the basin. If there 
were no atmosphere about the earth, the sun would 
not be so brought up above the horizon, and, as a 
consequence, it would set earlier and rise later by 
about a minute than it really does. This, of course, 
applies not merely to the sun, but to all celestial 
bodies that rise and set. Every planet and every 
star remains a shorter time below the horizon than 
it would if there were no atmosphere surrounding 
the earth. 

To apply this to the point we are discussing. 


76 


ON THE LUNAR ATMOSPHERE [chap. 


The moon in her orbital course across the heavens 
is continually passing before, or occulting, some of 
the stars that so thickly stud her apparent path. 
And when we see a star thus pass behind the lunar 
disc on one side and come out again on the other 
side, we are virtually observing the setting and 
rising of that star upon the moon. If, then, the 
moon had an atmosphere, it is clear, from analogy 
to the case of the earth, that the star must disappear 
later and reappear sooner than if it has no atmos¬ 
phere : just as a star remains too short a time 
below the earth’s horizon, or behind the earth, in 
consequence of the terrestrial atmosphere, so would 
a star remain too short a time behind the moon if 
an atmosphere surrounded that body. The point 
is settled in this way—The moon’s apparent dia¬ 
meter has been measured over and over again and 
is known with great accuracy; the rate of her 
motion across the sky is also known with perfect 
accuracy : hence it is easy to calculate how long the 
moon will take to travel across a part of the sky 
exactly equal in length to her own diameter. Sup¬ 
posing, then, that we observe a star pass behind 
the moon and out again, it is clear that, if there be 
no atmosphere, the interval of time during which it 
remains occulted ought to be exactly equal to the 


v.] 


EVIDENCE FROM OCCULTATION 


77 


computed time which the moon would take to pass 
over the star. If, however, from the existence of a 
lunar atmosphere, the star disappears too late and 
reappears too soon, as we have seen it would, these 
two intervals will not agree; the computed time 
will be greater than the observed time, and the 
difference, if any there be, will represent the amount 
of refraction the star’s light has sustained or suffered, 
and hence the extent of atmosphere it has had to 
pass through. 

Comparisons of these two intervals of time have 
been repeatedly made, the most recent and most 
extensive was executed under the direction of the 
Astronomer-Royal several years ago, and it was 
based upon no less than 296 occultation observa¬ 
tions. In this determination the measured or tele¬ 
scopic semidiameter of the moon was compared with 
the semidiameter deduced from the occultations, 
upon the above principle, and it was found.that the 
telescopic semidiameter was greater than the occul¬ 
tation semidiameter by two seconds of angular 
measurement or by about a thousandth part of the 
whole diameter of the moon. Sir George Airy, 
commenting on this result, says that it appears to 
him that the origin of this difference is to be sought 
in one of two causes. “ Either it is due to irradia- 


78 


ON THE LUNAR ATMOSPHERE [chap. 


tion # of the telescopic semidiameter, and I do not 
doubt that a part at least of the two seconds is to 
be ascribed to that cause; or it may be due to 
refraction by the moon’s atmosphere. If the whole 
two seconds were caused by atmospheric refraction 
this would imply a horizontal refraction of one 
second, which is only ~o part of the earth’s hori¬ 
zontal refraction. It is possible that an atmosphere 
competent to produce this refraction would not make 
itself visible in any other way.” This result accords 
well, considering the relative accuracy of the means 
employed, with that obtained a century ago by the 
French astronomer Du Sejour, who made a rigorous 
examination of the subject founded on observations 
of the solar eclipse of 1764. He concluded that 
the horizontal refraction produced by a possible 
lunar atmosphere amounted to 1"*5—a second and 
a half—or about -L of that produced by the earth’s 
atmosphere. The greater weight is of course to be 
allowed to the more recent determination in con- 

* Irradiation is an ocular phenomenon in virtue of which all 
strongly illuminated objects appear to the eye to be larger than 
they really are. The impression produced by light upon the 
retina appears to extend itself around the focal image formed by 
the lenses of the eye. It is from the effect of irradiation that a 
white disc on a black ground looks larger than a black disc of 
the same size on a white ground. 


V.] NO AIR OR WATER 79 

sideration of the large number of accurate observa¬ 
tions upon which it was based. 

But an atmosphere 2000 times rarer than our 
air can scarcely be regarded as an atmosphere at 
all. The contents of an air-pump receiver can 
seldom be rarefied to a greater extent than to about 
2-5 of the .density of air at the earth’s surface, with 
the best of pneumatic machines; and the lunar 
atmosphere, if it exist at all, is thus proved to be 
twice as attenuated as what we are accustomed to 
recognise as a vacuum. In discussing the physical 
phenomena of the lunar surface, we are, therefore, 
perfectly justified in omitting all considerations of 
an atmosphere, and adapting our arguments to the 
non-existence of such an appendage. 

And if there be no air upon the moon, we are 
almost forced to conclude that there can be no 
water; for if water covered any part of the lunar 
globe it must be vaporised under the influence of 
the long period of uninterrupted sunshine (upwards 
of 300 hours) that constitutes the lunar day, and 
would manifest itself in the form of clouds or mists 
obscuring certain parts of the surface. But, as we 
have already said, no such obliteration of details 
ever takes place; and, as we have further seen, no 
evidence of aqueous vapour is manifested upon the 


80 


ON THE LUNAR ATMOSPHERE [chap. 


occasion of spectrum observations. Since, then, 
the effects of watery vapour are absent, we are forced 
to conclude that the cause is absent also. 

Those parts of the moon which the ancient 
astronomers assumed, from their comparatively 
smooth and dusky appearance, to be seas, have long 
since been discovered to be merely extensive regions 
of less reflective surface material; for the telescope 
reveals to us irregularities and asperities covering 
well-nigh the whole of them, which asperities could 
not be seen if they were covered with water; unless, 
indeed, we admit the possibility of seeing to the 
bottom of the water, not only perpendicularly, but 
obliquely. Some observers have noticed features 
that have led them to suppose that water was at 
one time present upon the moon, and has left its 
traces in the form of appearances of erosive action 
in some parts. But if water ever existed, where is 
it now ? One writer, it is true, has suggested as 
possible, that whatever air, and we presume he 
would include whatever water also, the moon may 
possess, is hidden away in sublunarean caves and 
hollows; but even if water existed in these places 
it must sometimes assume the vapoury form, and 
thus make its presence known. 

Sir John Herschel pointed out that if any 






Fig. 13. 




Fig. 14. 





[To face page 81 


















v.] 


THE CRYOPHORUS 


81 


moisture exists upon the moon, it must be in a 
continual state of migration from the illuminated or 
hot, to the unilluminated or cold side of the lunar 
globe. The alternations of temperature, from the 
heat produced by the unmitigated sunshine of 14 
days’ duration, to the intensity of cold resulting 
from the absence of any sunshine whatever for an • 
equal period, must, he argued, produce an action 
similar to that of the cryophorus in transporting the 
lunar moisture from one hemisphere to the other. 
The cryophorus is a little instrument invented by 
the late Dr Wollaston; it consists of two bulbs of 
glass connected by a bent tube, in the manner shown 
in the annexed illustration, Fig. 13. One of the 
bulbs, A, is half-filled with water, and, all air being 
exhausted, the instrument is hermetically sealed, 
leaving nothing within but the water and the 
aqueous vapour which rises therefrom in the absence 
of atmospheric pressure. When the empty bulb, B, 
is placed in a freezing mixture, a rapid condensation 
of this vapour takes place within it, and as a con¬ 
sequence the water in the bulb A gives off more 
vapour. The abstraction of heat from the water, 
which is a natural consequence of this evaporation, 
causes it to freeze into a solid mass of ice. Now 
upon the moon the same phenomenon would occur 


82 


ON THE LUNAR ATMOSPHERE [chap. 


did the material exist there to supply it. In the 
accompanying diagram let A represent the illumin¬ 
ated or heated hemisphere of the moon, and B the 
dark or cold hemisphere; the former being probably 
at a temperature of 300° above, and the latter 200° 
below Fahrenheit’s zero. Upon the above principle, 
if moisture existed upon A it would become vapor¬ 
ised, and the vapour would migrate over to B, and 
deposit itself there as hoar-frost; it would, therefore, 
manifest itself to us while in the act of migrating by 
clouding or dimming the details about the boundary 
of the illuminated hemisphere. The sun, rising upon 
any point upon the margin of the dark hemisphere, 
would have to shine through a bed of moisture, 
and we may justly suppose, if this were the case, 
that the tops of mountains catching the first beams 
of sunlight would be tinged with colour, or be lit 
up at first with but a faint illumination, just as we 
see in the case of terrestrial mountains whose sum¬ 
mits catch the first, or receive the last beams of the 
rising or setting sun. Nothing of this kind is, how¬ 
ever, perceptible : when the solar rays tip the lofty 
peaks of lunar mountains, these shine at once with 
brilliant light, quite as vivid as any of those parts 
that receive less horizontal illumination, or upon 
which the sun is almost perpendicularly shining. 


V.] 


THE SURFACE OF THE MOON 


83 


All the evidence, then, that we have the means 
of obtaining, goes to prove that neither air nor 
water exists upon the moon. Two complicating 
elements affecting all questions relating to the 
geology of the terraqueous globe we inhabit may 
thus be dismissed from our minds while considering 
the physical features of the lunar surface. Fire on 
the one hand and water on the other, are the 
agents to which the configurations of the earth’s 
surface are referable: the first of these produced 
the igneous rocks that form the veritable founda¬ 
tions of the earth, the second has given rise to the 
superstructure of deposits that constitute the 
secondary and tertiary formations : were these last 
removed from the surface of our planet, so as to 
lay bare its original igneous crust, that crust, so far 
as reasoning can picture it to us, would probably 
not differ essentially from the visible surface of the 
moon. In considering the causes that have given 
birth to the diversified features of that surface, we 
may, therefore, ignore the influence of air and water 
action and confine our reasoning to igneous pheno¬ 
mena alone: our task in this matter, it is hardly 
necessary to remark, is materially simplified thereby. 


CHAPTER VI 

THE GENERAL ASPECT OF THE LUNAR SURFACE 

We have now reached that stage of our subject 
at which it behoves us to repair to the telescope for 
the purpose of examining and familiarising our¬ 
selves with the various classes.of detail that the 
lunar surface presents to our view. 

That the moon is not a smooth sphere of matter 
is a fact that manifested itself to the earliest 
observers. The naked eye perceives on her face 
spots exhibiting marked differences of illumination. 
These variations of light and shade, long before the 
invention of the telescope, induced the belief that 
she possessed surface irregularities like those that 
diversify the face of the earth, and from analogy it 
was inferred that seas and continents alternated 
upon the lunar globe. It was evident, from the 
persistence and invariability of the dusky markings, 
that they were not due to atmospheric peculiarities, 
but were veritable variations in the character or 

84 


chap, vi.] SUPERSTITION AND THE MOON 


85 


disposition of the surface material. Fancy made 
pictures of these unchangeable spots: untutored 
gazers detected in them the indications of a human 
countenance, and perhaps the earliest map of the 
moon was a rough reproduction of a man’s face, the 
eyes, nose, and mouth representing the more salient 
spots discernible upon the lunar disc. Others 
recognised in these spots the configuration of a 
human form, head, arms, and legs complete, which 
a French superstition that lingers to the present 
day held to be the image of Judas Iscariot trans¬ 
ported to the moon in punishment for his treason. 
Again, an Indian notion connects the lunar spots 
with a representation of a roebuck or a hare, and 
hence the Sanskrit names for the moon, mrigadhara , 
a roebuck-bearer, and ’ sa’sabhrit , a hare-bearer. 
Of these similitudes the one which has the best 
pretensions to a rude accuracy is that first men¬ 
tioned ; for the resemblance of the full moon to a 
human countenance, wearing a painful or lugubrious 
expression, is very striking. Our illustration of the 
full moon (Plate IY.) is derived from an actual 
photograph the relative intensities of light and 

* For the original photograph from which this plate was pro¬ 
duced, and for permission to reproduce it, we owe our acknow¬ 
ledgments to Warren De la Rue and Joseph Beck, Esquires. 


86 ASPECT OF LUNAR SURFACE [chap. 

shade are hence somewhat exaggerated; otherwise 
it represents the full moon very nearly as the naked 
eye sees it, and by gazing at the plate from a short 
distance, # the well-known features will manifest 
themselves, while they who choose may amuse 
themselves by arranging the markings in their 
imagination till they conform to the other appear¬ 
ances alluded to. 

We may remark in passing that by one sect of 
ancient writers the moon was supposed to be a kind 
of mirror, receiving the image of the earth and 
reflecting it back to terrestrial spectators. Hum¬ 
boldt affirmed that this opinion had been preserved 
to his day as a popular belief among the people of 
Asia Minor. He says, 44 I was once very much 
astonished to hear a very well educated Persian 
from Ispahan, who certainly had never read a Greek 
book, mention when I showed him the moon’s spots 
in a large telescope in Paris, this hypothesis as a 
widely diffused belief in his country: 4 What we 
see in the moon,’ said the Persian, 4 is ourselves; it 
is the map of our earth.’ ” Quite as extravagant 

* The proper distance for realising the conditions under 
which the moon itself is seen will be that at which our disc is 
just covered by a wafer about a quarter of an inch in diameter, 
held at arm’s length. This will subtend an angle of about half 
a degree, which is nearly the angular diameter of the moon. 


Plate IV.—Full Moon. 


[To Jace pagelSii. 










VI.] 


TIME OF AXIAL ROTATION 


87 


an idea, though perhaps a more excusable one, was 
that held by some ancient philosophers, to the 
effect that the spots on the moon were the shadows 
of opaque bodies floating in space between it and 
the sun. 

An observer watching the forms and positions 
of the lunar facemarks, from night to night and 
from lunation to lunation, cannot fail to notice the 
circumstance that they undergo no easily percep¬ 
tible change of position with respect to the circular 
outline of the disc; that in fact the face of the moon 
presented to our view is always the same, or very 
nearly so. If the moon had no orbital motion we 
should be led from the above phenomenon to con¬ 
clude that she had no axial motion, no movement of 
rotation ; but when we consider the orbital motion 
in connection with the permanence of aspect, we are 
driven to the conclusion — one, however* which 
superficial observers have some difficulty in recog¬ 
nising—that the moon has an axial rotation equal in 
period to her orbital revolution. Since the moon 
makes the circuit of her orbit in twenty-seven days 
and one-third (more exactly 27d. 7h. 43m. 11s.), it 
follows that this is the time of her axial rotation, as 
referred to the stars, or as it would be made out by 
an observer located at a fixed position in space out- 


88 ASPECT OF LUNAR SURFACE [chap. 

side the lunar orbit. But if referred to the sun this 
period appears different; because the moon while 
revolving round the earth, is with the earth, cir¬ 
culating around the sun. Suppose the three bodies, 
moon, earth, and sun, to be in a line at a certain 
period of a lunation, as they are at full moon : by 
the time the moon has completed her twenty-seven 
days’ journey around the earth, the latter will have 
moved along twenty-seven days’ march of its orbit, 
which is about twenty-seven degrees of celestial 
longitude: the sun will apparently be that much 
distant from a straight line passing through earth 
and moon, and the moon must therefore move for¬ 
ward to overtake the sun before she can assume the 
full phase again. She will take something over two 
days to do this; hence the solar period of her 
revolution becomes more than twenty-nine days (to 
be exact, 29d. 12h. 44m. 2s. *87). This is the 
length of a solar day upon the moon—the interval 
from one sunrise to another at any spot upon the 
equator of our satellite, and the interval between 
successive reappearances of the same phase to 
observers on the earth. The physical cause of the 
coincidence of times of rotation and revolution was 
touched upon in a previous chapter. 

We have said that the moon continuously pre- 


VI.] 


GALILEO 


89 


sents to us the same hemisphere. This is generally 
true, but not entirely so. Galileo, by long scrutiny, 
familiarised himself with every detail of the lunar 
disc that came within the limited grasp of his tele¬ 
scopes, and he recognised the fact that according as 
the position of the moon varied in the sky, so the 
aspect of her face altered to a slight degree; that 
certain regions at the edge of her disc alternately 
came in sight and receded from his view. He per¬ 
ceived, in fact, an apparent rocking to and fro of 
the globe of the moon; a sort of balancing or libra- 
tory motion. When the moon was near the horizon 
he could see spots upon her uppermost edge, which 
disappeared as she approached the zenith, or highest 
point of her nightly path; and as she neared this 
point, other spots, before invisible, came into view, 
near to what had been her lower edge. Galileo 
was not long in referring this phenomenon to its 
true cause. The centre of motion of the moon being 
the centre of the earth, it is clear that an observer 
on the surface of the latter looks down upon the 
rising moon as from an eminence, and thus he is en¬ 
abled to see more or less over or around her. As the 
moon increases in altitude, the line of sight gradu¬ 
ally becomes parallel to the line joining the observer 
and the centre of the earth, and at length he looks 


90 


ASPECT OF LUNAR SURFACE 


[chap. 


her full in the face: he loses the full view and 
catches another side face view as she nears the 
horizon in setting. This phenomenon, occurring as 
it does with a daily period, is known as the 
diurnal libration. 

But a kindred phenomenon presents itself in 
another period, and from another cause. The moon 
rotates upon her axis at a speed that is rigorously 
uniform. But her orbital motion is not uniform, 
sometimes it is faster, and at other times slower 
than its average rate. Hence, the angle through 
which she moves along her orbit in a given time, 
now exceeds, and now falls short of the angle 
through which she turns upon her axis. Her 
visible hemisphere thus changes to an extent de¬ 
pending upon the difference between these orbital 
and axial angles, and the apparent balancing thus 
produced is called the libration in longitude. Then 
there is a libration in latitude due to the circum¬ 
stance that the axis of the moon is not exactly 
perpendicular to the plane of her orbit; the effect 
of this inclination being, that we sometimes see a 
little more of the north than of the south polar 
regions of our satellite, and vice versa* 

* The libratory movement has been taken advantage of, at 
the suggestion of Sir Chas. Wheatstone, for producing stereo- 


THE MOON'S LIBRATIONS 


91 


vij 


The extent of the moon’s librations, taking them 
all and in combination into account, amounts to 

scopic photographs of the moon. In the early days of stereo¬ 
scopic photography the object to be photographed was placed 
upon a kind of turn-table, and, after a picture had been taken of 
it in one position, the table was turned through a small angle 
for the taking of the second picture ; the two placed side by side 
then represented the object as it would have been seen by two 
eyes widely separated, or whose visual rays inclined at an angle 
equal to that through which the table was turned ; and when 
the pictures were viewed through a stereoscope, they combined 
to produce the wonderful effect of solidity now familiar to every 
one. The moon, by its librations, imitates the turn-table move¬ 
ment ; and, from a large number of photographs of her, taken at 
different points of her orbit and at different seasons of the year, 
it is possible to select two which, while they exhibit the same 
phase of illumination, at the same time present the requisite 
difference in the points of view from which they are taken to 
give the effect of stereoscopicity when viewed binocularly. Mr 
De la Rue, the father of celestial photography, has been enabled 
to produce several such pairs of pictures from the vast collection 
of lunar photographs that he has accumulated. Any one of 
these pairs of portraits, when stereoscopically combined, repro¬ 
duces, to quote the words of Sir John Herschel, “ the spherical form 
just as a giant might see it whose stature were such that the 
interval between his eyes should equal the distance between the 
place where the earth stood when one view was taken, and that 
to which it would have to be removed (our moon being fixed) to 
get the other. Nothing can surpass the impression of real 
corporeal form thus conveyed by some of these pictures as taken 
by Mr De la Rue with his powerful reflector, the production of 
which (as a step in some sort taken by man outside of the planet 
he inhabits) is one of the most remarkable and unexpected 
triumphs of scientific art.” 


92 


ASPECT OF LUNAR SURFACE 


[chap. 


about seven degrees of arc of latitude or longitude 
upon the moon, both in the north-south and east- 
west directions. And taking into account the whole 
effect of them, we may conclude that our view of 
the moon’s surface, instead of being confined to one 
half, is extended really to about four-sevenths of 
the whole area of the lunar globe. The remaining 
three-sevenths must for ever remain a terra incog¬ 
nita to the habitants of this earth, unless, indeed, 
from some catastrophe which it would be wild 
fancy to anticipate, a period of rotation should be 
given to the moon different from that which it at 
present possesses. Some highly fanciful theorists 
have speculated upon the possible condition of the 
invisible hemisphere, and have propounded the 
absurd notion that the opposite side of the moon 
is hollow, or that the moon is a mere shell; others 
again have urged that the hidden half is more or 
less covered with water, and others again, that it is 
peopled with inhabitants. There is, however, no 
good reason for supposing that what we may call 
the back of the moon has a physical structure 
essentially different from the face presented to¬ 
wards us. So far as can be judged from the 
peeps that libration enables us to obtain, the 
same characteristic features (though of course 


VI.] 


GALILEO'S DISCOVERIES 


93 


with different details) prevail over the whole lunar 
surface. 

The speculative ideas held by the philosophers 
of the pre-telescopic age, touching the causes which 
produced the inequalities of light and shade upon 
the moon, received their coup de grace from the 
revelations of Galileo's glasses. Our satellite was 
one of the earliest objects, if not actually the first, 
upon which the Florentine turned his telescope; 
and he found that the inequalities upon her surface 
were due to differences in its configuration analogous 
to the continents and islands, and (as might then 
have been thought) the seas of our globe. He 
could trace, even with his moderate means, the 
semblance of mountain-tops upon which the sun 
shone while their lower parts were in shadow, of 
hills that were brightly illuminated upon their sides 
towards the sun, of brightly shining elevations, and 
deeply shadowed depressions, of smooth plains, and 
regions of mountainous ruggedness. He saw that 
the boundary of sunlight upon the moon was not a 
clearly defined line, as it would be if the lunar globe 
were a smooth sphere, as the Aristotelians had 
asserted, but that the terminator was uneven and 
broken into an irregular outline. From these 
observations the Florentine astronomer concluded 


94 ASPECT OF LUNAR SURFACE [chap. 

that the lunar world was covered not only with 
mountains like our globe, but with mountains 
whose heights far surpassed those existing upon the 
earth, and whose forms were strangely limited to 
circularity. 

Galileo’s best telescopes magnified only some 
thirty times, and the views which he thus obtained, 
must have been similar to those exhibited by the 
smaller photographs of the moon produced in late 
years by Mr De la Rue and now familiar to the 
scientific public. Of course there is in the natural 
moon as viewed with a small telescope a vivid 
brilliancy which no art can imitate, and in photo¬ 
graphs especially there is a tendency to exaggera¬ 
tion of the depths of shade in a lunar picture. This 
arises from the circumstance that various regions of 
the moon do not impress a chemically sensitised 
plate as they impress the retina of the eye. Some 
portions, notably the so-called “ seas ” of the moon, 
which to the eye appear but slightly duller than the 
brighter parts, give off so little actinic light that 
they appear as nearly black patches upon a photo¬ 
graph, and thus give an undue impression of the 
relative brightness of various parts of the lunar 
surface. Doubtless by sufficient exposure of the 
plate in the camera-telescope the dark patches 


VI.] 


TELESCOPES 


95 


might be rendered lighter, but in that case the more 
strongly illuminated portions, which after all are 
those most desirable to be preserved, would be lost 
by the effect which photographers understand as 
“ solarisation.” 

In speaking of a view of the moon with a 
magnifying power of thirty, it is necessary to bear 
in mind that the visible features will differ consider¬ 
ably with the diameter of the object-glass of the 
telescope to which this power is applied. The 
same details would not be seen alike with the same 
power upon an object-glass of 10 inches diameter 
and one of 2 inches. The superior illumination of 
the image in the former case would bring into view 
minute details that could not be perceived with the 
smaller aperture. He who would for curiosity wish 
to see the moon, or any other object, as Galileo saw 
it, must use a telescope of the same size and char¬ 
acter in all respects as Galileo’s: it will not do to 
put his magnifying power upon a larger telescope. 
With large telescopes, and low powers used upon 
bright objects like the moon, there is a blinding 
flood of light which tends to contract the pupil of 
the eye and prevent the passage of the whole of the 
pencil of rays coming through the eye-piece. 
Although this last result may be productive of no 


96 ASPECT OF LUNAR SURFACE [chap. 

inconvenience, it is clearly a waste of light, and it 
points to a rule that the lowest power that a tele¬ 
scope should bear is that which gives a pencil of 
light equal in diameter to the pupil of the eye under 
the circumstances of brightness attendant upon the 
object viewed. In observing faint objects this 
point assumes more importance, since it is then 
necessary that all available light should enter the 
pupil. The thought suggests itself that an artificial 
enlargement of the pupil, as by a dose of bella¬ 
donna, might be of assistance in searching for faint 
objects, such as nebulae and comets : but we prefer 
to leave the experiment for those to try who pursue 
that branch of astronomical observation. 

A merely cursory examination of the moon with 
the low power to which we have alluded is sufficient 
to show us the more salient features. In the first 
place we cannot help being struck with the immense 
preponderance of circular or craterform asperities, 
and with the general tendency to circular shape 
which is apparent in nearly all the lunar surface 
markings; for even the larger regions known as the 
“seas” and the smaller patches of the same char¬ 
acter seem to repeat in their outlines the round form 
of the craters. It is at the boundary of sunlight on 
the lunar globe that we see these craterform spots 


VI.] 


KEPLER’S HYPOTHESIS 


97 


to the best advantage, as it is there that the rising 
or setting snn casts long shadows over the lunar 
landscape, and brings elevations and asperities into 
bold relief. They vary greatly in size, some are so 
large as to bear an estimable proportion to the 
moon’s diameter, and the smallest are so minute as 
to need the most powerful telescopes and the finest 
conditions of atmosphere to perceive them. It is 
doubtful whether the smallest of them have ever 
been seen, for there is no reason to doubt that there 
exist countless numbers that are beyond the reveal¬ 
ing powers of our finest telescopes. 

From the great number and persistent character 
of these circumvallations, Kepler was led to think 
that they were of artificial construction. He re¬ 
garded them as pits excavated by the supposed 
habitants of the moon to shelter themselves from 
the long and intense action of the sun. Had he 
known their real dimensions, of which we shall 
have to speak when we come to describe them more 
in detail, he would have hesitated in propounding 
such a hypothesis; nevertheless it was, to a certain 
extent, justified by the regular and seemingly un¬ 
natural recurrence of one particular form of struc¬ 
ture, the like of which is, too, so seldom met with as 
a structural feature of the surface of our own globe^ 


98 ASPECT OF LUNAR SURFACE [chap. 

The next most striking features, revealed by a 
low telescopic power upon the moon, are the seem¬ 
ingly smooth plains that have the appearance of 
dusky spots, and that collectively cover a consider¬ 
able portion—about two-thirds—of the entire disc. 
The larger of these spots retain the name of seas, 
the term having been given when they were sup¬ 
posed to be watery expanses, and having been 
retained, possibly to avoid the confusion inevitable 
from a change of name, after the existence of water 
upon the moon was disproved. Following the same 
order of nomenclature, the smaller spots have 
received the appellations of lakes, hays, and fens. 
We see that many of these “seas” are partially 
surrounded by ramparts or bulwarks which, under 
closer examination, and having regard to their 
real magnitude, resolve themselves into immense 
mountain chains. The general resemblance in form 
which the bulwarked plains thus exhibit to the 
circular craters of large size, would lead us to 
suppose that the two classes of objects had the 
same formative origin, but when we take into 
account the immense size of the former, and the 
process by which we infer the latter to have been 
developed, the supposition becomes untenable. 

Another of the prominent features which we 


VI.] 


GENERAL FEATURES 


99 


notice as highly curious, and in some phases of the 
moon—at about the time of full—the most remark¬ 
able of all, are certain bright lines that appear to 
radiate from some of the more conspicuous craters, 
and extend for hundreds of miles around. No 
selenological formations have so sorely puzzled 
observers as these peculiar streaks, and a great deal 
of fanciful theorising has been bestowed upon them. 
As we are now only glancing at the moon, we do 
not enter upon explanations concerning them or 
any other class of details; all such will receive due 
consideration in their proper order in succeeding 
chapters. 

We thus see that the classes of features observ¬ 
able upon the moon are not great in number : they 
may be summed up as craters and their central 
cones, mountain chains , with occasional isolated 
peaks, smooth plains , with more or less of irregu¬ 
larity of surface, and bright radiating streaks. But 
when we come to study with higher powers the 
individual examples of each class we meet with 
considerable diversity. This is especially the case 
with the craters, which appear under very numerous 
variations of the one order of structure, viz., the 
ring-form. A higher telescopic power shows us 
that not only do these craters exist of all magnitudes 
ILefC. 


100 ASPECT OF LUNAR SURFACE [chap. 

within a limit of largeness, but seemingly with no 
limit of smallness, but that in their structure and 
arrangement they present a great variety of points 
of difference. Some are seen to be considerably 
elevated above the surrounding surface, others are 
basins hollowed out of that surface and with low 
surrounding ramparts; some are merely like walled 
plains or amphitheatres with flat plateaux, while 
the majority have their lowest point of hollowness 
considerably below the general level of the sur¬ 
rounding surface; some are isolated upon the plains, 
others are aggregated into a thick crowd, and over¬ 
lapping and intruding upon each other; some have 
elevated peaks or cones in their centres, and some 
are without these central cones, while the plateaux 
of others again contain several minute craters 
instead; some have their ramparts whole and 
perfect, others have them breached or malformed, 
and many have them divided into terraces, especially 
on their inner sides. 

In the plains, what with a low power appeared 
smooth as a water surface, becomes, under greater 
magnification, a rough and furrowed area, here 
gently undulated and there broken into ridges and 
declivities, with now and then deep rents or cracks 
extending for miles and spreading like river-beds 


VI.] 


DIVERSITY OF COLOUR 


101 


into numerous ramifications. Craters of all sizes and 
classes are scattered over the plains; these appear 
generally of a different tint from the surround¬ 
ing surface, for the light reflected from the plains 
has been observed to be slightly tinged with colour. 
The tint is not the same in all cases : one large sea 
has a dingy greenish tinge, others are merely grey, 
and some others present a pale reddish hue. The 
cause of this diversity of colour is mysterious; it 
has been supposed to indicate the existence of 
vegetation of some sort; but this involves conditions 
that we know do not exist. 

The mountains, under higher magnification, do 
not present such diversity of formation as the 
craters, or at least the points of difference are not 
so apparent; but they exhibit a plentiful variety of 
combinations. There are a few perfectly isolated 
examples that cast long shadows over the plains on 
which they stand like those of a towering cathedral 
in the rising or setting sun. Sometimes they are 
collected into groups, but mostly they are connected 
into stupendous chains. In one of the grandest of 
these chains, it has been estimated that a good 
telescope will show 3000 mountains clustered to¬ 
gether, without approach to symmetrical order. 
The scenery which they would present, could we 


102 ASPECT OF LUNAR SURFACE [chap. 

get any other than the “ bircPs-eye view ” to which 
we are confined, must be imposing in the extreme, 
far exceeding in sublime grandeur anything that 
the Alps or the Himalayas offer; for while on the 
one hand the lunar mountains equal those of the 
earth in altitude, the absence of an atmosphere, and 
consequently of the effects produced thereby, must 
give rise to alternations of dazzling light and black 
depths of shade combining to form panoramas of 
wild scenery that, for want of a parallel on earth, 
we may well call unearthly. But we are debarred 
the pleasure of actually contemplating such pictures 
by the circumstance that we look down upon the 
mountain tops and into the valleys, so that the 
great height and close aggregation of the peaks 
and hills are not so apparent. To compare the 
lunar and terrestrial mountain scenery would be 
“to compare the different views of a town seen 
from the car of a balloon with the more interesting 
prospects by a progress through the streets.” Some 
of the peculiarities of the lunar scenery we have, 
however, endeavoured to realise in a subsequent 
chapter. 

A high power gives us little more evidence than 
a low one upon the nature of the long bright streaks 
that radiate from some of the more conspicuous 


VI.] 


BRIGHT STREAKS 


103 


craters, but it enables us to see that those streaks 
do not arise from any perceptible difference of -level 
of the surface—that they have no very definite 
outline, and that they do not present any sloping 
sides to catch more sunlight, and thus shine 
brighter, than the general surface. Indeed, one 
great peculiarity of them is that they come out 
most forcibly where the sun is shining perpendi¬ 
cularly upon them ; hence they are best seen where 
the moon is at full, and they are not visible at all 
at those regions upon which the sun is rising or 
setting. W e also see that they are not diverted by 
elevations in their path, as they traverse in their 
course craters, mountains, and plains alike, giving 
a slight additional brightness to all objects over 
which they pass, but producing no other effect 
upon them. To employ a commonplace simile, 
they look as though, after the whole surface 
of the moon had assumed its final configuration, 
a vast brush charged with a whitish pigment 
had been drawn over the globe in straight lines 
radiating from a central point, leaving its trail 
upon everything it touched, but obscuring 
nothing. 

Whatever may be the cause that produces this 
brightness of certain parts of the moon without 


104 ASPECT OF LUNAR SURFACE [chap. 

reference to configuration of surface, this cause has 
not been confined to the formation of the radiating 
lines, for we meet with many isolated spots, streaks, 
and patches of the same bright character. Upon 
some of the plains there are small areas and lines 
of luminous matter possessing peculiarities similar 
to those of the radiating streaks, as regards visibility 
with the high sun, and invisibility when the solar 
rays fall upon them horizontally. Some of the 
craters also are surrounded by a kind of aureole 
of this highly reflective matter. A notable speci¬ 
men is that called Linne , concerning which a great 
hue and cry about change of appearance and in¬ 
ferred continuance of volcanic action on the moon 
was raised some years ago. This object is an 
insignificant little crater of about a mile or two in 
diameter, in the centre of an ill-defined spot of the 
character referred to, and about eight or ten miles 
in diameter. With a low sun the crater alone is 
visible by its shadow; but as the luminary rises 
the shadow shortens and becomes all but invisible, 
and then the white spot shines forth. These alter¬ 
nations, complicated by variations of atmospheric 
condition, and by the interpretations of different 
observers, gave rise to statements of somewhat 
exaggerated character to the effect that consider- 


VI.] 


SCOPE OF THE TELESCOPE 


105 


able changes, of the nature of volcanic eruptions, 
were in progress in that particular region of the 
moon. 

In the foregoing remarks we have alluded 
somewhat indefinitely to high powers; and an 
inquiring but unastronomical reader may reason¬ 
ably demand some information upon this point. It 
might have been instructive to have cited the 
various details that may be said to come into view 
with progressive increases of magnification. But 
this would be an all but impossible task, on account 
of the varying conditions under which all astro¬ 
nomical observations must necessarily be made. 
When we come to delicate tests, there are no 
standards of telescopic power and definition. As¬ 
suming the instrument to be of good size and high 
optical character, there is yet a powerful influencer 
of astronomical definition in the atmosphere and 
its variable state. Upon two-thirds of the clear 
nights of a year the finest telescopes cannot be 
used to their full advantage, because the minute 
flutterings resulting from the passage of the rays 
of light through moving strata of air of different 
densities are magnified just as the image in the 
telescope is magnified, till all minute details are 
blurred and confused, and only the grosser features 


106 ASPECT OF LUNAR SURFACE [chap. 

are left visible. And supposing the telescope and 
atmosphere in good state, there is still an important 
point, the state of the observer’s eye, to be con¬ 
sidered. After all it is the eye that sees, and the 
best telescopic assistance to an untrained eye is of 
small avail. The eye is as susceptible of education 
and development as any other organ; a skilful and 
acute observer is to a mere casual gazer what a 
watchmaker would be to a ploughman, a miniature 
painter to a whitewasher. This fact is not generally 
recognised; no man would think of taking in hand 
an engraver’s burin, and expecting on the instant 
to use it like an adept, or of going to a smithy and 
without previous preparation trying to forge a 
horse-shoe. Yet do folks enter observatories with 
uneducated eyes, and expect at once to realise all 
the wonderful things that their minds have pictured 
to themselves from the perusal of astronomical 
books. We have over and over again remarked 
the dissatisfaction which attends the first looks of 
novices through a powerful telescope. They anti¬ 
cipate immediately beholding wonders, and they 
are disappointed at finding how little they can see, 
and how far short the sight falls of what they had 
expected. Courtesy at times leads them to express 
wonder and surprise, which it is easy to see is not 


VI.] 


LIMITS OF MAGNIFICATION 


107 


really felt, but sometimes honesty compels them to 
give expression to their disappointment. This arises 
from the simple fact that their eyes are not fit for the 
work which is for the moment imposed upon them ; 
they know not what to look for, or how to look for 
it. The first essay at telescopic gazing, like first 
essays generally, serves but to teach us our incapa¬ 
bility. 

To a tutored eye a great deal is visible with a 
comparatively low power, and practised observers 
strive to use magnifying powers as low as possible, 
so as to diminish, as far as may be, the evils arising 
from an untranquil atmosphere. With a power so 
small as 30 or 40, many exceedingly delicate details 
on the moon are visible to an eye that is familiar 
with them under higher powers. With 200 we 
may say that every ordinary detail will come out 
under favourable conditions; but when minute 
points of structure, mere nooks and corners as it 
were, are to be scrutinised, 300 may be used with 
advantage. Another hundred diameters almost 
passes the practical limit. Unless the air be not 
merely fine, but superfine, the details become 
“ clothy ” and tremulous; the extra points brought 
out by the increased power are then only caught 
by momentary glimpses, of which but a very few 


108 ASPECT OF LUNAR SURFACE [chap. 

are obtained during a lengthy period of persistent 
scrutiny. We may set down 250 as the most use¬ 
ful, and 350 the utmost effective power that can be 
employed upon the particular work of which we 
are treating. Could every detail on the moon be 
thoroughly and reliably represented as this amount 
of magnification shows it, the result would leave 
little to be wished for. 

But it may be asked by some, what is the 
absolute effect of such powers as those we have 
spoken of, in bringing the moon apparently nearer 
to our eyes? and what is the actual size of the 
smallest object visible under the most favourable 
circumstances ? A linear mile upon the moon corre¬ 
sponds to an angular interval of 0*87 of a second; 
this refers to regions about the centre of the disc ; 
near the circumference the foreshortening makes a 
difference, very great as the edge is approached. 
Perhaps the smallest angle that the eye can with¬ 
out assistance appreciate is half a minute ; that is 
to say, an object that subtends to the eye an arc of 
less than half a minute can scarcely be seen. # Since 

* This is a point of some uncertainty. Dr Young stated 
( Lectures , vol. ii., p. 575) that “a minute is perhaps nearly the 
smallest interval at which two objects can be distinguished, 
although a line subtending only a tenth of a minute in breadth 
may sometimes be perceived as a single object.” 


VI.] MINUTEST VISIBLE LUNAR OBJECTS 109 


there are 60 seconds in a minute, it follows that we 
must magnify a spot a second in diameter upon the 
moon thirty times before we can see it; and since 
a second represents rather more than a mile, really 
about 2000 yards, on the moon, as seen from the 
earth, the smallest object visible with a power of 
30 will be this number of yards in diameter or 
breadth. To see an object 200 yards across, we 
should require to magnify it 300 times, and this 
would only bring it into view as a point; 20 yards 
would require a power of 3000, and 1 yard 60,000 
to effect the same thing. Since, as we have said, 
the highest practicable power with our present tele¬ 
scopes, and at ordinary terrestrial elevations, is 
350, or for an extreme say 400, it is evident that 
the minutest lunar object or detail of which we can 
perceive as a point must measure about 150 yards : 
to see the form of an object, so as to discriminate 
whether it be round or square, it would require to 
be probably twice this size; for it may be safely 
assumed that we cannot perceive the outline of an 
object whose average breadth subtends a less angle 
than a minute. 

Arago put this question into another shape :— 
The moon is distant from us 237,000 miles (mean). 
A magnifying power of a thousand would show us 


110 ASPECT OF LUNAR SURFACE [chap. vi. 

the moon as if she were distant 237 miles from the 
naked eye. 

2000 would bring her within 118 miles. 

4000 „ „ „ 59 „ 

6000 „ „ „ 39 „ 

Mont Blanc is visible to the naked eye from Lyons, 
at the distance of about 100 miles; so that to see 
the mountains of the moon as Mont Blanc is seen 
from Lyons would require the impracticable power 
of 2500. 


CHAPTER YII 


TOPOGRAPHY OF THE MOON 

It is scarcely necessary to seek the reasons which 
prompted astronomers, soon after the invention of 
the telescope, to map the surface features of the 
moon. They may have considered it desirable to 
record the positions of the spots upon her disc, for 
the purpose of facilitating observations of the 
passage of the earth’s shadow over them in lunar 
eclipses; or they may have been actuated by a 
desire to register appearances then existing, in 
order that if changes took place in after years these 
might be readily detected. Scheiner was one of 
the earliest of lunar cartographers; he worked about 
the middle of the seventeenth century; but his 
delineations were very rough and exaggerated. 
Better maps—the best of the time, according to an 
old authority—were engraved by one Mellan, about 
the years 1634 or 1635. At about the same epoch 
Langreen and Hevelius were working upon the 

in 


112 TOPOGRAPHY OF THE MOON [chap. 

same subject. Langreen executed some thirty 
maps of portions of the moon, and introduced the 
practice of naming the spots after philosophers and 
eminent men. Hevelius spent several years upon 
his task, the results of which he published in a 
bulky volume containing some 50 maps of the 
moon in various phases, and accompanied by 500 
pages of letterpress. He rejected Langreen’s 
system of nomenclature, and called the spots after 
the seas and continents of the earth to which he 
conceived they bore resemblance. Riccioli, another 
selenographer, whose map was compiled from ob¬ 
servations made by Grimaldi, restored LangreeiTs 
nomenclature, but he confined himself to the names 
of eminent astronomers, and his system has gained 
the adhesion of the map-makers of later times. 
Cassini prepared a large map from his own obser¬ 
vations, and it was engraved about the year 1692. 
It appears to have been regarded as a standard 
work, for a reduced copy of it was repeatedly issued 
with the yearly volumes of the Connaissance des 
Temps (the Nautical Almanac of France) some time 
after its publication. These small copies have no 
great merit: the large copper plate of the original 
was, we are told by Arago, who received the state¬ 
ment from Bouvard, sold to a brazier by a director 


VII.] LUNAR CARTOGRAPHERS 113 

of the French Government Printing Office who 
thought proper to disembarrass the stores of that 
establishment, by ridding them of what he con¬ 
sidered lumber! La Hire, Mayer, and Lambert 
followed, during the succeeding century, in this 
branch of astronomical delineation. At the com¬ 
mencement of the present century, the subject 
was very earnestly taken up by the indefatigable 
Schroeter, who, although he does not appear to 
have produced a complete map, produced a topo¬ 
graph of the moon in a large series of partial maps 
and drawings of special features. Schroeter was a 
fine observer, but his delineations show him to 
have been an indifferent draughtsman. Some of his 
drawings are but the rudest representations of the 
objects he intended to depict; many of the bolder 
features of conspicuous objects are scarcely recog¬ 
nisable in them. A bad artist is as likely to mislead 
posterity as a bad historian, and it cannot be sur¬ 
prising if observers of this or future generations, 
accepting Schroeter’s drawings as faithful represen¬ 
tations, should infer from them remarkable changes 
in the lunar details. It is much to be regretted 
that Schroeter’s work should be thus depreciated. 
Lohrman of Dresden, was the next cartographer of 
the moon; in 1824 he put forth a small but very 


114 TOPOGRAPHY OF THE MOON [chap. 

excellent map of 15 inches diameter, and pub¬ 
lished a book of descriptive text, accompanied by 
sectional charts of particular areas. His work, 
however, was eclipsed by the great one which we 
owe to the joint energy of MM. Beer and Maedler, 
and which represents a stupendous amount of ob¬ 
serving work carried on during several years prior 
to 1836, the date of their publication. The long 
and patient labour bestowed upon their map and 
upon the measures on which it depends, deserve 
the highest praise which those conversant with the 
subject can bestow, and it must be very long before 
their efforts can be superseded. 

Beer and Maedler’s map has a diameter of 37 
inches : it represents the phase of the moon visible 
in the condition of mean libration. The details 
were charted by a careful process of triangulation. 
The disc was first divided into “ triangles of the 
first order,” the points of which (conspicuous 
craters) were accurately laid down by reference to 
the edges of the disc: one hundred and seventy-six 
of these triangles, plotted accurately upon an ortho¬ 
graphic projection of the hemisphere, formed the 
reliable basis for their charting work. From these 
a great number of “points of the second order” 
were laid down, by measuring their distance and 




Fig. 15. 


[To face page 115. 



VII.] 


BEER AND MAEDLER 


115 


angle of position with regard to points first estab¬ 
lished. The skeleton map thus obtained was filled 
up by drawings made at the telescope: the 
diameters of the measurable craters being deter¬ 
mined by the micrometer. 

Beer and Maedler also measured the heights of 
one thousand and ninety-five lunar mountains and 
crater-summits: the resulting measures are given 
in a table contained in the comprehensive text-book 
which accompanies their map. These heights are 
found by one of^two methods, either by measuring 
the length of the shadow which the object casts 
under a known elevation of the sun above its hori¬ 
zon, or by measuring the distance between the 
illuminated point of the mountain and the “ termi¬ 
nator ” in the following manner. In the annexed 
figure (Fig. 15) let the circle represent the moon 
and m a mountain upon it: let s a be the line of 
direction of the sun’s rays, passing the normal 
surface of the moon at a and just tipping the 
mountain top. a will be the terminator, and there 
will be darkness between it and the star-like 
mountain summit m. The distance between a and 
m is measured : the distance a b is known, for it is 
the moon’s radius. And since the line s m is a 
tangent to the circle, the angle b a m is a right angle. 


116 TOPOGRAPHY OF THE MOON [chap. 

We know the length of its two sides ab, am, and 
we can therefore by the known properties of the 
right-angled triangle find the length of the liypo- 
tlienuse bm : and since bm is made up of the radius 
ba plus the mountain height, we have only to sub¬ 
tract the moon’s radius from the ascertained whole 
length of the hypothenuse and we have the height 
of the mountain. MM. Beer and Maedler exhibited 
their measures in French toises : in the heights we 
shall have occasion to quote, these have been turned 
into English feet, upon the assumption that the 
toise is equal to 6.39 English feet. The nomencla¬ 
ture of lunar features adopted by Beer and Maedler 
is that introduced by Riccioli: mountains and 
features hitherto undistinguished were named by 
them after ancient and modern philosophers, in 
Riccioli’s system, and occasionally after terrestrial 
features. Some minute objects in the neighbour¬ 
hood of large and named ones were included under 
the name of the large one and distinguished by 
Greek or Roman letters. 

The excellent map resulting from the arduous 
labours of these astronomers is simply a map: it 
does not pretend to be a picture. The asperities 
and depressions are symbolised by a conventional 
system of shading, and no attempt is made to ex- 


VII.] 


MAP OF THE MOON 


117 


hibit objects as they actually appear in the tele¬ 
scope. A casual observer comparing details on the 
map with the same details on the moon itself would 
fail to identify or recognise them except where the 
features are very conspicuous. Such an observer 
would be struck by the shadows by which the lunar 
objects reveal themselves: he would get to know 
them mostly by their shadows, since it is mainly by 
those that their forms are revealed to a terrestrial 
observer. But such a map as that under notice 
indicates no shadows, and objects have to be 
identified upon it rather by their positions with 
regard to one another or to the borders of the moon 
than by any notable features they actually present 
to view. This inconvenience occurred to us in our 
early use of Beer and Maedler’s chart, and we were 
induced to prepare for ourselves a map in which 
every object is shown somewhat, if imperfectly, as 
it actually appears at some period of a lunation. 
This was done by copying Beer and Maedler’s out¬ 
lines and filling them up by appropriate shading. 
To do justice to our task we enlarged our map to a 
diameter of six feet. Upon a circle of this diameter 
the positions and dimensions of all objects were 
laid down from the German original. Then from 
our own observations we depicted the general aspect 


118 


TOPOGRAPHY OF THE MOON 


[chap. 


of each object: and we so adjusted the shading 
that all objects should be shown under about the 
same angle of illumination—a condition which is 
never fulfilled upon the moon itself, but which we 
consider ourselves justified in exhibiting for the 
purpose of conveying a fair impression of how the 
various lunar objects actually appear at some one or 
other part of a lunation. 

The picture-map thus produced has been photo¬ 
graphed to a size convenient for this work : and in 
order to make it available for the identification of 
such objects—chiefly lunar craters—as we may have 
occasion to refer to, we have prepared a skeleton 
map which includes the more conspicuous objects 
of that nature. The progressive numbers in the 
annexed list refer to the skeleton map, and the 
description of the object to which they are annexed 
will be found on pp. 121-151. 


No. Name. 

1. Newton. 

2. Short. 

3. Simpelius. 

4. Manzinus. 

5. Moretus. 

6. Gruemberger. 

7. Casatus. 

8. Klaproth. 

9. Wilson. 

10. Kircher. 


No. Name. 

11. Bettinus. 

12. Blancanus. 

13. Clavius. 

14. Scheiner. 

15. Zuchius. 

16. Segner. 

17. Bacon. 

18. Nearchus. 

19. Vlacq. 

20. Hommel. 


No. Name. 

21. Licetus. 

22. Maginus. 

23. Longomon- 

tanus. 

24. Schiller. 

25. Phocylides. 

26. Wargentin. 

27. Inghirami. 

28. Schickard. 

29. Wilhelm I. 







A aw/ ojA 


■('A 3 1 T! [J) * IV I\r aHOLDIJ AXVJ K03.TV OJ y 

‘NOOJV 3H1 30 3VIM N0X333NS 












































* 













- 





s, 



























. 








Plate V.—Picture Map of the Moon. 


[To face page 119-. 



VII.] 


KEY TO THE SKELETON MAP 


119 


No. Name. 

30. Tycho. 

31. Saussure. 

32. Stoefler. 

33. Maurolycus. 

34. Barocius. 

35. Fabricius. 

36. Metius. 

37. Fernelius. 

38. Heinsius. 

39. Hainzel. 

40. Bouvard. 

41. Piazzi. 

42. Ramsden. 

43. Capuanus. 

44. Cichus. 

45. Wurzelbauer. 

46. Gauricus. 

47. Hell. 

48. Walter. 

49. Nonius. 

50. Riccius. 

51. Rheita. 

52. Furnerius. 

53. Stevinus. 

54. Hase. 

55. Snell. 

56. Borda. 

57. Neander. 

58. Piccolomini. 

59. Pontanus. 

60. Poisson. 

61. Aliacensis. 

62. Werner. 

63. Pitatus. 

64. Hesiodus. 

65. Mercator. 

66. Vitello. 

67. Fourier. 

68. Lagrange. 


No. Name. 

69. Vieta. 

70. Doppelmayer. 

71. Campanus. 

72. Kies. 

73. Purbach. 

74. La Caille. 

75. Playfair. 

76. Azophi. 

77. Sacrobosco. 

78. Fracastorius. 

79. Santbech. 

80. Petavius. 

81. Wilhelm 

Humboldt. 

82. Polybius. 

83. Geber. 

84. Arzachael. 

85. Thebit. 

86. Bullialdus. 

87. Hippalus. 

88. Cavendish. 

89. Mersenius. 

90. Gassendi. 

91. Lubiniezky. 

92. Alpetragius. 

93. Airy. 

94. Almanon. 

95. Catharina. 

96. Cyrillus. 

97. Theophilus. 

98. Colombo. 

99. Vendelinus. 

100. Langreen. 

101. Goclenius. 

102. Guttemberg. 

103. Isidorus. 

104. Capella. 

105. Kant. 

106. Descartes. 


No. Name. 

107. Abulfeda. 

108. Parrot. 

109. Albategnius. 

110. Alphons. 

111. Ptolemy. 

112. Herschel. 

113. Davy. 

114. Guerike. 

116. Bonpland. 

117. Lalande. 

118. Reaumur. 

120. Letronne. 

121. Billy. 

122. Fontana. 

123. Hansteen. 

124. Damoiseau. 

125. Grimaldi. 

126. Flamsteed. 

127. Landsberg. 

128. Moesting. 

129. Deambrel. 

130. Taylor. 

131. Messier. 

132. Maskelyne. 

133. Sabine. 

134. Ritter. 

135. Godin. 

136. Soemmering. 

137. Schroeter. 

138. Gambart. 

139. Reinhold. 

140. Encke. 

141. Hevelius. 

142. Riccioli. 

143. Lohrman. 

144. Cavalerius. 

145. Reiner. 

146. Kepler. 

147. Copernicus. 


[chap. 


120 TOPOGRAPHY OF THE MOON 


No. Name. 

148. Stadius. 

149. Pallas. 

150. Triesnecker. 

151. Agrippa. 

152. Arago. 

153. Taruntius. 

154. Apollonius. 

155. Schubert. 

156. Firmicus. 

157. Silberschlag. 

158. Hyginus. 

159. Ukert. 

160. Boscovich. 

161. Ross. 

162. Proclus. 

163. Picard. 

164. Condorcet. 

165. Pliny or 

Menelaus. 

167. Manilius. 

168. Erastothenes. 

169. Gay Lussac. 

170. Tobias Mayer. 

171. Marius. 

172. Olbers. 

173. Vasco deGama. 

174. Seleucus. 


No. Name. 

175. Herodotus. 

176. Aristarchus. 

177. La Hire. 

178. Pytheas. 

179. Bessel. 

180. Vitruvius. 

181. Maraldi. 

182. Macrobius. 

183. Cleomides. 

184. Roemer. 

185. Littrow. 

186. Posidonius. 

187. Geminus. 

188. Linnaeus. 

189. Autolycus. 

190. Aristillus. 

191. Archimedes. 

192. Timocharis. 

193. Lambert. 

194. Diophantus. 

195. Delisle. 

196. Briggs. 

197. Lichtenberg. 

199. Calippus. 

200. Cassini. 

201. Gauss. 

202. Messala. 


No. Name. 

203. Struve. 

204. Mason. 

205. Plana. 

206. Burg. 

207. Baily. 

208. Eudoxus. 

209. Aristotle. 

210. Plato. 

211. Pico. 

212. Helicon. 

213. Maupertuis. 

214. Condamine. 

215. Bianchini. 

216. Sharp. 

217. Mairan. 

218. Gerard. 

219. Repsold. 

220. Pythagoras. 

221. Fontenelle. 

222. Timaeus. 

223. Epigenes. 

224. Gartner. 

225. Thales. 

226. Strabo. 

227. Endymion. 

228. Atlas. 

229. Hercules. 


The strong family likeness pervading the craters 
of the moon renders it unnecessary that we should 
attempt a description of each one of them or even 
of one in twenty. We have, however, thought that 
a few remarks upon the salient features of a few of 
the most important may be acceptable in explana¬ 
tion of our illustrative plates; and what we have 


VII.] 


COPERNICUS 


121 


to say of the few may be taken as representative of 
the many. 

COPERNICUS, 147. Plate VI. 

This may deservedly be considered as one of the 
grandest and most instructive of lunar craters. 
Although its vast diameter (46 miles) is exceeded by 
others, yet, taken as a whole, it forms one of the 
most impressive and interesting objects of its class. 
Its situation, near the centre of the lunar disc, 
renders all its wonderful details, as well as those of 
its immediately surrounding objects, so conspicuous 
as to establish it as a very favourite object. Its 
vast rampart rises to upwards of 12,000 feet above 
the level of the plateau, nearly in the centre of 
which stands a magnificent group of cones, three 
of them attaining the height of upwards of 2400 
feet. 

The rampart is divided by concentric segmental 
terraced ridges, which present every appearance of 
being enormous landslips, resulting from the crush¬ 
ing of their over-loaded summits, which have slid 
down in vast segments and scattered their debris 
on to the plateau. Corresponding vacancies in the 
rampart may be observed from whence these 
prodigious masses have broken away. The same 


122 TOPOGRAPHY OF THE MOON [chap. 

may be noticed, although in a somewhat modified 
degree, around the exterior of the rampart. In 
order to approach a realisation of the sublimity and 
grandeur of this magnificent example of a lunar 
volcanic crater, our reader will do well to endeavour 
to fix his attention on its enormous magnitude and 
attempt to establish in his mind’s eye a correct 
conception of the scale of its details as well as its 
general dimensions, which, as they so prodigiously 
transcend those of the largest terrestrial volcanic 
craters, require that our ideas as to magnitude of 
such objects should be, so to speak, educated upon 
a special standard. It is for this reason we are 
anxious our reader, when examining our illustra¬ 
tions, should constantly refer the objects represented 
in them to the scale of miles appended to each plate, 
otherwise a just and true conception of the grandeur 
of the objects will escape him. 

Copernicus is specially interesting, as being 
evidently the result of a vast discharge of molten 
matter which has been ejected at the focus or 
centre of disruption of an extensively upheaved 
portion of the lunar crust. A careful examination 
of the crater and the district around it, even to the 
distance of more than 100 miles on every side, will 
supply unmistakable evidence of the vast extent 


Plate VI.—Copernicus. 


; o 5 o 

MILES 


It 


20 30 SO 60 

Scale 


70 so 


[To face page 122. 





* * 



VII.] 


BRIGHT STREAKS 


123 


and force of the original disruption, manifested by a 
wonderfully complex reticulation of bright streaks 
which diverge in every direction from the crater as 
their common centre. These streaks do not appear 
on our plate, nor are they seen upon the moon 
except at and near the full phase. They show 
conspicuously, however, by their united lustre on 
the full moon, Plate IY. Every one of those bright 
streaks, we conceive, is a record of what was origin¬ 
ally a crack or chasm in the solid crust of the moon, 
resulting from some vastly powerful upheaving 
agency over the site of whose focus of energy 
Copernicus stands. The cracking of the crust must 
have been followed by the ejection of subjacent 
molten matter up through the reticulated cracks; 
this, spreading somewhat on either side of them, 
has left these bright streaks as a visible record of 
the force and extent of the upheaval; while at the 
focus of disruption from whence the cracks diverge, 
the grand outburst appears to have taken place, 
leaving Copernicus as its record and result. 

Many somewhat radial ridges or spurs may be 
observed leading away from the exterior banks of 
the great rampart. These appear to be due to the 
more free egress which the extruded matter would 
find near the focus of disruption. The spur-ridges 


124 TOPOGRAPHY OF THE MOON [chap. 

may be traced fining away for fully 100 miles on 
all sides, until they become such delicate objects as 
to approach invisibility. Several vast open chasms 
or cracks may be observed around the exterior of 
the rampart. They appear to be due to some 
action subsequent to the formation of the great 
crater—probably the result of contraction on the 
cooling of the crust, or of a deep-seated upheaval 
long subsequent to that which resulted in the for¬ 
mation of Copernicus itself, as they intersect 
objects of evidently prior formation. 

Under circumstances specially favourable for 
“ fine vision,’’ for upwards of 70 miles on all sides 
around Copernicus, myriads of comparatively minute 
but perfectly-formed craters may be observed. The 
district on the south-east side is specially rich in 
these wonderfully thickly scattered craters, which 
we have reason to suppose stand over or upon the 
reticulated bright streaks; but, as the circumstances 
of illumination which are requisite to enable us to 
detect the minute craters are widely adverse to those 
which render the bright streaks visible, namely, 
nearly full moon for the one and gibbous for the 
other, it is next to impossible to establish the fact 
of coincidence of the sites of the two by actual 
simultaneous observation. 


VII.] 


GASSENDI 


125 


At the east side of the rampart, multitudes of 
these comparatively minute craters may also be 
detected, although not so closely crowded together 
as those on the west side; but among those on 
the east may be seen myriads of minute promi¬ 
nences roughening the surface; on close scrutiny 
these are seen to be small mounds of extruded 
matter which, not having been ejected with 
sufficient energy to cause the erupted material to 
assume the crater form around the vent of ejec¬ 
tion, have simply assumed the mound form so well 
known to be the result of volcanic ejection of 
moderate force. 

Were we to select a comparatively limited por¬ 
tion of the lunar surface abounding in the most 
unmistakable evidence of volcanic action in every 
variety that can characterise its several phases, we 
could not choose one yielding in all respects such 
instructive examples as Copernicus and its immedi¬ 
ate surroundings. 

GASSENDI, 90. Plate VII. 

An interesting crater about 54 miles in dia¬ 
meter ; the height of the most elevated portion of 
the surrounding wall from the plateau being about 
9600 feet. The centre is occupied by a group of 


126 TOPOGRAPHY OF TPIE MOON [chap. 

conical mountains, three of which are most con¬ 
spicuous objects and rise to nearly 7000 feet above 
the level of the plateau. As in other similar cases, 
these central mountains are doubtless the result of 
the expiring effort of the eruption which had formed 
the great circular wall of the crater. The plateau 
is traversed by several deep cracks or chasms nearly 
one mile wide. 

Both the interior and exterior of the wall of 
the crater are terraced with the usual segmental 
ridges or landslips. A remarkable detached portion 
of the interior bank is to be seen on the east side, 
while on the west exterior of the wall may be seen 
an equally remarkable example of an outburst of 
lava subsequent to the formation of the wall or 
bank of the crater; it is of conical form and cannot 
fail to secure the attention of a careful observer. 

Interpolated on the north wall of the crater may 
be seen a crater of about 18 miles diameter which 
has burst its bank in towards the great crater, upon 
whose plateau the lava appears to have discharged 
itself. 

The neighbourhood of Gassendi is diversified by 
a vast number of mounds and long ridges of exu- 
dated matter, and also traversed by enormous 
chasms and cracks, several of which exceed one 



Plate VII.—Gassendi. 


70 S 0 

10 

20 

30 


SI 


* ■ 1 


i 

- 1 

1 


M I LC5 

Scale 


[To face page 126 













VII.] 


EUDOXUS AND ARISTOTLE 


127 


mile wide and are fully 100 miles in length, and, 
as is usual with such cracks, traverse plain and 
mountain alike, disregarding all inequalities. 

Numbers of small craters are scattered around ; 
the whole forming an interesting and instructive 
portion of the lunar surface. 

EUDOXUS, 208, and ARISTOTLE, 209. Plate VIII. 

Two gigantic craters, Eudoxus being nearly 
35 miles in diameter and upwards of 11,000 feet 
deep, while Aristotle is about 48 miles in diameter, 
and about 10,000 feet deep (measuring from the 
summit of the rampart to the plateau). These two 
magnificent craters present all the true volcanic 
characteristics in a remarkable degree. The out¬ 
sides, as well as the insides of their vast surround¬ 
ing walls or banks display on the grandest scale 
the landslip feature, the result of the over-piling of 
the ejected material, and the consequent crushing 
down and crumbling of the substructure. The 
true eruptive character of the action which formed 
the craters is well evinced by the existence of the 
groups of conical mountains, which occupy the 
centres of their circular plateaux, since these coni¬ 
cal mountains, there can be little doubt, stand over 


128 TOPOGRAPHY OF THE MOON [chap. 

what were once the vents from whence the ejected 
matter of the craters was discharged. 

On the west side of these grand craters may be 
seen myriads of comparatively minute ones (we use 
the expression “ comparatively minute,” although 
most of them are fully a mile in diameter). So 
thickly are these small craters crowded together, 
that counting them is totally out of the question; 
in our original notes we have termed them “ Froth 
craters” as the most characteristic description of 
their aspect. 

The exterior banks of Aristotle are char¬ 
acterised by radial ridges or spurs : these are most 
probably the result of the flowing down of great 
currents of very fluid lava. To the east of the 
craters some very lofty mountains of exudation may 
be seen, and immediately beyond them an extensive 
district of smaller mountains of the same class, so 
thickly crowded together as under favourable 
illumination to present a multitude of brilliant 
points of light contrasted by intervening deep shade. 
On the west bank of Aristotle a very perfect crater 
may be seen, 27 miles in diameter, having all the 
usual characteristic features. 

About 40 miles to the east of Eudoxus there is 
a fine example of a crack or fissure extending fully 



Plate VIII. — Aristotle and Eudoxus. 

to SO to 2 O .30 -f 0 SO 60 

■miJiuju_ i ~ i _ > . _| 

miles 

Scale 


[To face, page 128. 






VII.] 


TRIESNECKER 


129 


50 miles—30 miles through a plain, and the remain¬ 
ing 20 miles cutting through a group of very lofty 
mountains. This great crack is worthy of attention, 
as giving evidence of the deep-seated nature of the 
force which occasioned it inasmuch as it disregards 
all surface impediments, traversing plain and group 
of mountains alike. 

There are several other features in and around 
these two magnificent craters well worthy of careful 
observation and scrutiny, all of them excellent 
types of their respective classes. 

TRIESNECKER, 150. Plate IX. 

A fine example of a normal lunar volcanic crater, 
having all the usual characteristic features in great 
perfection. Its diameter is about 20 miles, and it 
possesses a good example of the central cone and 
also of interior terracing. 

The most notable feature, however, in connection 
with this crater, and on account of which we have 
chosen it as a subject for one of our illustrations, is 
the very remarkable display of chasms or cracks 
which may be seen to the west side of it. Several 
of these great cracks obviously diverge from a small 
crater near the west external bank of the great one, 
and they subdivide or branch out, as they extend 

i 


130 


TOPOGRAPHY OF THE MOON 


[chap. 


from the apparent point of divergence, while they 
are crossed or intersected by others. These cracks 
or chasms (for their width merits the latter appella¬ 
tion) are nearly one mile broad at the widest part, 
and after extending to fully 100 miles, taper away 
till they become invisible. Although they are not 
test objects of the highest order of difficulty, yet to 
see them with perfect distinctness requires an in¬ 
strument of some perfection and all the conditions 
of good vision. When such are present, a keen 
and practised eye will find many details to rivet its 
attention, among which are certain portions of the 
edges of these cracks or chasms which have fallen 
in and caused interruptions to their continuity. 

THEOPHILUS, 97 ; CYRILLUS, 96 ; CATHARINA, 95. 

Plate X. 

These three magnificent craters form a very 
conspicuous group near the middle of the south-east 
quarter of the lunar disc. 

Their respective diameters and depths are as 
follows:— 

Theophilus, 64 miles diameter; depth of plateau 
form summit of crater wall, 16,000 feet; central 
cone, 5200 feet high. 

Cyrillus, 60 miles diameter; depth of plateau 



Plate IX.—Triesnecker. 


JO 5 O ro 20 00 40 so 

1 ■ ■ I ■ I.. .. I _I_I_I_1_I 

miles 

SCALE 


[To face page 130. 







VII.] TUEOPHlLtJS} CYRILLUS, CATHARINA 131 

from summit of crater wall, 15,000 feet ; central 
cone, 5800 feet high. 

Catharina, 65 miles diameter; depth of plateau 
from summit of crater wall, 13,000 feet; centre of 
plateau occupied by a confused group of minor 
craters and debris. 

Each of these three grand craters is full of 
interesting details, presenting in every variety the 
characteristic features which so fascinate the atten¬ 
tion of the careful observer of the moon’s wonder¬ 
ful surface, and affording unmistakable evidence of 
the tremendous energy of the volcanic forces which 
at some inconceivably remote period piled up such 
gigantic formations. 

Theophilus by its intrusion within the area of 
Cyrillus shows in a very striking manner that it is 
of comparatively more recent formation than the 
latter crater. There are many such examples in 
other parts of the lunar disc, but few of so very 
distinct and marked a character. 

The flanks or exterior banks of Theophilus, 
especially those on the west side, are studded with 
apparently minute craters, all of which when care¬ 
fully scrutinised are found to be of the true volcanic 
type of structure ; and minute as they are, by com¬ 
parison, they would to a beholder close to them 


132 TOPOGRAPHY OF THE MOON [chap. 

appear as very imposing objects; but so gigantic 
are the more notable craters in the neighbourhood, 
that we are apt to overlook what are in themselves 
really large objects. It is only by duly training the 
mind, as we have previously urged, so as ever to 
keep before us the vast scale on which the volcanic 
formations of the lunar surface are displayed, that 
we can do them the justice which their intrinsic 
grandeur demands. We trust that our illustrations 
may in some measure tend to educate the mind’s 
eye, so as to derive to the full the tranquil enjoy¬ 
ment which results from the study of the manifesta¬ 
tion of one of the Creator’s most potent agencies in 
dealing with the materials of His worlds, namely, 
volcanic force. So rich in wonderful features and 
characteristic details is this magnificent group and 
its neighbourhood, that a volume might be filled in 
the attempt to do justice, by description, to objects 
so full of suggestive subject for study. 

PTOLEMY, 111 ; ALPHONS, 110; ARZACHAEL, 84, 
ETC. Plate XI. 

The portion of the lunar surface comprised 
within the limits of this illustration being situated 
nearly in the centre of the moon’s disc, is very 
favourably placed for revealing the multitude of 


Plate X.—Theophilus, Cyrillus, and Catharina. 

JO S 0 JO 20 30 40 SO 6 0 7 0 80 

1 1 _L_I_l_1_I_1 



[To face page 132. 















p 

































% 


9 








V 










I 





























«• 






r t. 







. * 




4 


* 

- 



* ^ 


. 














J 










VII.] PTOLEMY, ALPHONS, ARZACHAEL 133 

interesting features and details therein represented. 
They consist of every variety of volcanic crater from 
“ Ptolemy,” whose vast rampart is eighty-six miles 
diameter, down to those whose dimensions are, 
comparatively, so minute as to render them at the 
extreme limits of visibility. 

Alphons and Arzachael, two of the next largest 
craters in our illustration, situated immediately 
above Ptolemy, are sixty and fifty-five miles in 
diameter respectively, and are possessed, in a re¬ 
markable degree, of all the distinctive characteristic 
features of lunar craters, having magnificent central 
cones, lofty ragged ramparts, together with very 
striking manifestations of landslip formations as 
appear in the great segmental terraces within their 
ramparts, together with several minor craters inter¬ 
polated on their plateau. “Alphons,” the middle 
crater of this fine group, has its plateau specially 
distinguished by several cracks or chasms fully one 
mile wide, the direction or “strike” of which coin¬ 
cide in a very remarkable manner with several other 
similar cracks which form conspicuous features 
among the multitude of interesting details comprised 
within the limits of our illustration,—the most 
notable of these is an enormous straight cliff 
traversing the diameter of a low-ridged circular 


134 TOPOGRAPHY OF THE MOON [chap. 

formation, seen in the upper right-hand corner of 
our plate. This great cliff is sixty miles long and 
from 1000 to 2000 feet high; it is a well-known 
object to lunar observers, and has been termed 
“■The Railway,” on account of its straightness as 
revealed by the distinct shadow projected by it on 
the plateau when seen under its sunrise aspect. The 
face of this vast cliff', although generally straight, is 
seen, when minutely scrutinised, to be somewhat 
serrated in its outline, while on its upper edge may 
be detected some very minute but perfectly formed 
craters. The existence of this remarkable cliff 
appears to be due either to an upheaval or a down- 
sinking of portion of the surface of the circular area 
across whose diameter it extends. 

To the right-hand side of the cliff are two small 
craters from the side of which a fine example of a 
crack may be detected passing through in its course 
a low dome-formed hill; this crack is parallel to 
the cliff, having in that respect the same general 
strike or parallel direction which so remarkably 
distinguishes the other cracks observable in this 
portion of the moon’s surface. 

On the left hand of this great cliff is situated a 
coneless crater, named “ Thebit,” on the right-hand 
rampart of which may be observed two small 



Plate XI. — Ptolemy, Alphons, Arzachael, etc. 


iQ S O 
MILES 


TO 


■20 




fO 30 60 7p 


Sfj _90 


Scale 


[To face page 134, 







VII.] 


PLATO 


135 


craters, the lesser of which is 275 miles diameter 
and has a central cone. We specially remark this 
fact, as it is the smallest lunar crater but one, in 
which we have, with perfect distinctness, detected 
a central cone. Not but that many smaller lunar 
craters exist possessed of this unmistakable evidence 
of their volcanic origin; but so minute are the 
specks of light which the central cones of such small 
craters reflect, that they, for that reason, most pro¬ 
bably fail to reveal themselves. 

PLATO, 210. Plate XII. 

This crater, besides being a conspicuous object 
on account of its great diameter, has many interest¬ 
ing details in and around it requiring a fine in¬ 
strument and favourable circumstances to render 
them distinctly visible. The diameter of the crater 
is 70 miles ; the surrounding wall or rampart varies 
in height from 4000 to upwards of 8000 feet, and 
is serrated with noble peaks which cast their black 
shadows across the plateau in a most picturesque 
manner, like the towers and spires of a great 
cathedral. Reference to our illustration will con¬ 
vey a very fair idea of this interesting appearance. 
On the north-east inside of the circular wall or 
rampart may be observed a fine example of land- 


136 TOPOGRAPHY OF THE MOON [chap. 

slip, or sliding down of a considerable mass of the 
interior side of the crater’s wall. The landslip nature 
of this remarkable detail is clearly established by 
the fact of the bottom edge of the downslipped 
mass projecting in towards the centre of the plateau 
to a considerable extent. Other smaller landslip 
features may be seen, but none on so grand and 
striking a scale as the one referred to. A number 
of exceedingly minute craters may be detected on 
the surface of the plateau. The plateau itself is 
remarkable for its low reflective power, which 
causes it to look like a dingy spot when Plato is 
viewed with a small magnifying power. The ex¬ 
terior of the crater wall is remarkable for the 
rugged character of its formation, and forms a great 
contrast in that respect to the comparatively smooth 
unbroken surface of the plateau, which by the way 
is devoid of a central cone. The surrounding 
features and objects indicated in our illustration 
are of the highest interest, and a few of them 
demand special description. 

THE VALLEY OF THE ALPS. Plate XII. 

This remarkable object lies somewhat diagon¬ 
ally to the west of Plato; when seen with a low 
magnifying power (80 to 100), it appears as a rut 



Plate XII. 


Plato, the Valley of the Alps, Pico, etc. 


Q $ to 
MILES 


20 


30 4o SO 

-j- -J- ., *■ 

Scale 




*70 


00 


[To face page 136 
















VII.] 


THE VALLEY OF THE AITS 


137 


or groove tapering towards each extremity. It 
measures upwards of 75 miles long by about six 
miles wide at the broadest part. When examined 
under favourable circumstances, with a magnifying 
power of from 200 to 300, it is seen to be a vast 
flat-bottomed valley bordered by gigantic mountains, 
some of which attain heights upwards of 10,000 
feet; towards the south-east of this remarkable 
valley, and on both sides of it, are groups of isolated 
mountains, several of which are fully 8000 feet high. 
This flat-bottomed valley, which has retained the 
integrity of its form amid such disturbing forces as 
its immediate surroundings indicate, is one of the 
many structural enigmas with which the lunar 
surface abounds. To the north-west of the valley a 
vast number of isolated mounds or small mountains 
of exudation may be seen; so numerous are they 
as to defy all attempts to count them with anything 
like exactness; and among them, a power of 200 
to 300 will enable an observer, under favourable 
circumstances, to detect vast numbers of small but 
perfectly-formed craters. 

PICO, 211. Plate XII. 

This is one of the most interesting examples of 
an isolated volcanic “ mountain of exudation,” and 


138 


TOPOGRAPHY OF THE MOON 


[CHAP. 


it forms a very striking object when seen under 
favourable circumstances. Its height is upwards 
of 8000 feet, and it is about three times as long at 
the base as it is broad. The summit is cleft into 
three peaks, as may be ascertained by the three- 
peaked shadow it casts on the plain. Five or six 
minute craters of very perfect form may be detected 
close to the base of this magnificent mountain. 
There are several other isolated peaks or mountains 
of the same class within 30 or 40 miles of it which 
are well worthy of careful scrutiny, but Pico is the 
master of the situation, and offers a glorious subject 
for realising a lunar day-dream in the mind’s eye, 
if we can only by an effort of imagination conceive 
its aspect under the fiercely brilliant sunshine by 
which it is illuminated, contrasted with the intensely 
black lunar heavens studded with stars shining with 
a steady brightness of which, by reason of our 
atmosphere intervening, we can have no adequate 
conception save by the aid of a well-directed imagi¬ 
nation. 


TYCHO, 30. Plate XIII. 

This magnificent crater, which occupies the 
centre of the crowded group in our Plate, is 54 
miles in diameter, and upwards of 16,000 feet deep, 


VII.] 


TYCHO 


139 


from the highest ridge of the rampart to the surface 
of the plateau, whence rises a grand central cone 
5000 feet high. It is one of the most conspicuous 
of all the lunar craters, not so much on account of 
its dimensions as from its occupying the great focus 
of disruption from whence diverge those remark¬ 
able bright streaks, many of which may be traced 
over 1000 miles of the moon’s surface, disregarding 
in their course all interposing obstacles. There is 
every reason to conclude that Tycho is an instance 
of a vast disruptive action which rent the solid 
crust of the moon into radiating fissures, which 
were subsequently occupied by extruded molten 
matter, whose superior luminosity marks the course 
of the cracks in all directions from the crater as 
their common centre of divergence. So numerous 
are these bright streaks when examined by the aid 
of the telescope, and they give to this region of the 
moon’s surface such an extra degree of luminosity, 
that, when viewed as a whole, their locality can be 
distinctly seen at full moon by the unassisted eye 
as a bright patch of light on the southern portion 
of the disc. (See Plate IV.) The causative origin 
of the streaks is discussed and illustrated in 
Chapter XI. 

The interior of this fine crater presents striking 


140 TOPOGRAPHY OF THE MOON [chap. 

examples of the concentric terrace-like formations 
that we have elsewhere assigned to vast landslip 
actions. Somewhat similar concentric terraces may 
be observed in other lunar craters; some of these, 
however, appear to be the results of some temporary 
modification of the ejective force, which has caused 
the formation of more or less perfect inner ram¬ 
parts : what we conceive to be true landslip terraces 
are always distinguished from these by their more 
or less fragmentary character. 

On reference to Plate IV., showing the full 
moon, a very remarkable and special appearance 
will be observed in a dingy district or zone immedi¬ 
ately surrounding the exterior of the rampart of 
Tycho, and of which we venture to hazard what 
appears to us a rational explanation: namely, that 
as Tycho may be considered to have acted as a sort 
of safety-valve to the rendering and ejective force 
which caused, in the first instance, the cracking of 
this vast portion of the moon’s crust—the molten 
matter that appears to have been forced up through 
these cracks, on finding a comparatively free exit 
by the vent of Tycho, so relieved the district 
immediately around him as to have thereby re¬ 
duced, in amount, the exit of the molten matter, 
and so left a zone comparatively free from the 



Plate XIII.—Tycho and its surroundings. 


JO 5 0 >0 20 30 40 SO 60 70 60 

iui.IUUI---1--'- 1 - 1 -* — 1 


Ml 1.6* 


Scale 


[To face page 140. 






















VII.] 


WARGENTIN 


141 


extruded lava- which, according to our view of the 
subject, came up simultaneously through the in¬ 
numerable fissures, and, spreading sideways along 
their courses, left everlasting records of the original 
positions of the radiating cracks in the form of the 
bright streaks which we now behold. 

WARGENTIN, 26. Plate XVIII. 

This object is quite unique of its kind—a crater 
about 53 miles across, that to all appearance has 
been filled to the brim with lava that has been left 
to consolidate. There are evidences of the remains 
of a rampart, especially on the south-west portion 
of the rim. The general aspect of this extra¬ 
ordinary object has been not unaptly compared to 
a “thin cheese.” The terraced and rutted exterior 
of the rampart has all the usual characteristic 
details of the true crater. The surface of the high 
plateau is marked by a few ridges branching from 
a point nearly in its centre, together with some 
other slight elevations and depressions; these, how¬ 
ever, can only be detected when the sun’s rays fall 
nearly parallel to the surface of the plateau. 

To the north of this interesting object is the 
magnificent ring formation Schickard, whose vast 
diameter of 123 miles contrasts strikingly with that 


142 


TOPOGRAPHY OP THE MOON [chap. 


of the sixteen small craters within his rampart, 
and equally so with a multitude of small craters 
scattered around. There are many objects of 
interest on the portion of the lunar surface included 
within our illustration, but as they are all of the 
usual type, we shall not fatigue the attention of 
our readers by special descriptions of them. 

ARISTARCHUS, 176, and HERODOTUS, 175. Plate XIV. 

These two fine examples of lunar volcanic craters 
are conspicuously situated in the north-east quarter 
of the moon’s disc. Aristarchus has a circular 
rampart nearly 28 miles diameter, the summit of 
which is about 7500 feet above the surface of the 
plateau, while its height above the general surface 
of the moon is 2600 feet. A central cone having 
several subordinate peaks completes the true vol¬ 
canic character of this crater: its rampart banks, 
both outside and inside, have fine examples of 
the segmental crescent-shaped ridges or landslips, 
which form so constant and characteristic a feature 
in the structure of lunar craters. Several very 
notable cracks or chasms may be seen to the north 
of these two craters. They are contorted in a very 
unusual and remarkable manner, the result probably 


VII.] 


ARISTARCHUS, HERODOTUS 


143 


of the force which formed them having to encounter 
very varying resistance near the surface. 

Some parts of these chasms gape to the width 
of two to three miles, and when closely scrutinised 
are seen to be here and there partly filled by masses 
which have fallen inward from their sides. Several 
smaller craters are scattered around, which, together 
with the great chasms and neighbouring ridges, 
give evidence of varied volcanic activity in this 
locality. We must not omit to draw attention to 
the parallelism or general similarity of “ strike ” in 
the ridges of extruded matter; this appearance has 
special interest in the eyes of geologists, and is well 
represented in our illustration. 

Aristarchus is specially remarkable for the 
extraordinary capability which the material forming 
its interior and rampart banks has of reflecting 
light. Although there are many portions of the 
lunar surface which possess the same property, yet 
few so remarkably as in the case of Aristarchus, 
which shines with such brightness, as compared 
with its immediate surroundings, as to attract the 
attention of the most unpractised observer. Some 
have supposed this appearance to be due to active 
volcanic discharge still lingering on the lunar 
surface, an idea in which, for reasons to be duly 


144 


TOPOGRAPHY OF THE MOON 


[chap. 


adduced, we have no faith. Copernicus, in the 
remarkable bright streaks which radiate from it, and 
Tycho also, as well as several other spots, are appar¬ 
ently composed of material very nearly as highly re¬ 
flective as that of Aristarchus. But the comparative 
isolation of Aristarchus, as well as the extraordinary 
light-reflecting property of its material, renders it 
especially noticeable, so much so as to make it 
quite a conspicuous object when illuminated only 
by earth-light, when but a slender crescent of the 
lunar disc is illuminated, or when, as during a lunar 
eclipse, the disc of the moon is within the shadow 
of the earth and is lighted only by the rays refracted 
through the earth’s atmosphere. 

There are no features about Herodotus of any 
such speciality as to call for remark, except it be 
the breach of the north side of its rampart by the 
southern extremity of a very remarkable contorted 
crack or chasm, which to all appearance owes its 
existence to some great disruptive action subse¬ 
quent to the formation of the crater. 


WALTER, 48, and adjacent Intrusive Craters. 

Plate XIX. (facing p. 188). 

This plate represents a southern portion of the 
moon’s surface, measuring 170 by 230 miles. It 



Plate XIV.—Aristarchus and Herodotus. 

70 S 0 10 20 30 <*0 SO 

1 .. I . I I ..*■>!., .. I- • ■ 

MILES 

SCA L E 


[To face page 144, 







VII.] 


WALTER 


145 


includes upwards of 200 craters of all dimensions, 
from Walter, whose rampart measures nearly 70 
miles across, down to those of such small apparent 
diameter as to require a well-practised eye to detect 
them. In the interior of the great crater, Walter, a 
remarkable group of small craters may be observed 
surrounding its central cone, which in this instance 
is not so perfectly in the centre of the rampart as is 
usually the case. The number of small craters 
which we have observed within the rampart is 20, 
exclusive of those on the rampart itself. The 
entire group represented in the Plate suggests in a 
striking manner the wild scenery which must 
characterise many portions of the lunar surface; 
the more so if we keep in mind the vast proportions 
of the objects which they comprise, upon which 
point we may remark that the smallest crater repre¬ 
sented in this Plate is considerably larger than that 
of Vesuvius. 

ARCHIMEDES, 191; AUTOLYCUS, 189; ARISTILLUS, 
190, and the APENNINES. Plate XXII. (facing p. 210). 

This group of three magnificent craters, together 
with their remarkable surroundings, especially in¬ 
cluding the noble range of mountains termed the 
Apennines, forms on the whole one of the most 

K 


146 TOPOGRAPHY OF THE MOON [chap. 

striking and interesting portions of the lunar surface. 
If the reader is not acquainted with what the tele¬ 
scope can reveal as to the grandeur of the effect of 
sunrise on this very remarkable portion of the 
moon’s surface, he should carefully inspect and 
study our illustration of it; and if he will pay due 
regard to our previously repeated suggestion con¬ 
cerning the attached scale of miles, he will, should 
he have the good fortune to study the actual objects 
by the aid of a telescope, be well prepared to realise 
and duly appreciate the magnificence of the scene 
which will be presented to his sight. 

Were we to attempt an adequate detailed 
description of all the interesting features comprised 
within our illustration, it would, of itself, fill a 
goodly volume; as there is included within the 
space represented every variety of feature which so 
interestingly characterises the lunar surface. All 
the more prominent details are types of their class; 
and are so favourably situated in respect to almost 
direct vision, as to render their nature, forms, and 
altitudes above and depths below the average 
surface of the moon most distinctly and impres¬ 
sively cognisable. 

Archimedes is the largest crater in the group; 
it has a diameter of upwards of 52 miles, measuring 


ARCHIMEDES 


VII.] 


147 


from summit to summit of its vast circular ram, 
part or crater wall, the average height of which, 
above the plateau, is about 4300 feet; but some 
parts of it rise considerably higher, and, in conse¬ 
quence, cast steeple-like shadows across the plateau 
when the sun’s rays are intercepted by them at a 
low angle. The plateau of this grand crater is 
devoid of the usual central cone. Two compara¬ 
tively minute but beautifully-formed craters may 
be detected close to the north-east interior side of 
the surrounding wall of the great crater. Both 
outside and inside of the crater wall may be seen 
magnificent examples of the landslip subsidence of 
its overloaded banks; these landslips form vast 
concentric segments of the outer and inner circum¬ 
ference of the great circular rampart, and doubtless 
belong to its era of formation. Two very fine 
examples of cracks, or chasms, may be observed 
proceeding from the opposite external sides of the 
crater, and extending upwards of 100 miles in each 
direction; these cracks, or chasms, are fully a mile 
wide at their commencement next the crater, and 
narrow away to invisibility at their further ex¬ 
tremity. Their course is considerably crooked, and 
in some parts they are partially filled by masses of 
the material of their sides, which have fallen inward 


148 


TOPOGRAPHY OF THE MOON [chap. 


and partially choked them. The depths of these 
enormous chasms must be very great, as they pro¬ 
bably owe their existence to some mighty upheaving 
action, which there is every reason to suppose 
originated at a profound depth, since the general 
surface on each side of the crater does not appear 
to be disturbed as to altitude, which would have 
been the case had the upheaving action been at a 
moderate depth beneath. We would venture to 
ascribe a depth of not less than ten miles as the 
most moderate estimate of the profundity of these 
terrible chasms. If the reader would realise the 
scale of them, let him for a moment imagine himself 
a traveller on the surface of the moon coming upon 
one of them, and finding his onward progress 
arrested by the sudden appearance of its vast black 
yawning depths; for by reason of the angle of his 
vision being almost parallel to the surface, no 
appearance of so profound a chasm would break 
upon his sight until he came comparatively close to 
its fearful edge. Our imaginary lunar traveller 
would have to make a very long detour, ere he 
circumvented this terrible interruption to his pro¬ 
gress. If the reader will only endeavour to realise 
in his mind’s eye the terrific grandeur of a chasm a 
mile wide and of such dark profundity as to be, to 


VII.] 


A PROFOUND CHASM 


149 


all appearance, fathomless—portions of its rugged 
sides fallen in wild confusion into the jaws of the 
tortuous abyss, and catching here and there a ray 
of the sun sufficient only to render the darkness of 
the chasm more impressive as to its profundity—he 
will, by so doing, learn to appreciate the romantic 
grandeur of this, one of the many features which the 
study of the lunar surface presents to the careful 
observer, and which exceed in sublimity the wildest 
efforts of poetic and romantic imagination. The 
contemplation of these views of the lunar world are, 
moreover, vastly enhanced by especial circumstances 
which add greatly to the impressiveness of lunar 
scenery, such as the unchanging pitchy-black aspect 
of the heavens and the death-like silence which 
reigns unbroken there. 

These digressions are, in some respects, a fore- 
stalment of what we have to say by-and-by, and so 
far they are out of place; but with the illustration 
to which the above remarks refer placed before the 
reader, they may, in some respects, enhance the 
interest of its examination. 

The upper portion of our illustration is occupied 
by the magnificent range of volcanic mountains 
named after our Apennines, extending to a length 
of upwards of 450 miles. This mountain group 


150 TOPOGRAPHY OF THE MOON [chap. 

rises gradually from a comparatively level surface 
towards the south-west, in the form of innumerable 
comparatively small mountains of exudation, which 
increase in number and altitude towards the north¬ 
east, where they culminate and suddenly terminate 
in a sublime range of peaks, whose altitude and 
rugged aspect must form one of the most terribly 
grand and romantic scenes which imagination can 
conceive. The north-east face of the range termin¬ 
ates abruptly in an almost vertical precipitous face, 
and over the plain beneath intense black steeple or 
spire-like shadows are cast, some of which at sun¬ 
rise extend fully 90 miles, till they lose themselves 
in the general shading due to the curvature of the 
lunar surface. Nothing can exceed the sublimity 
of such a range of mountains, many of which rise 
to heights of 18,000 to 20,000 feet at one bound 
from the plane at their north-east base. The most 
favourable time to examine the details of this 
magnificent range is from about a day before first 
quarter to a day after, as it is then that the general 
structure of the range as well as the character of 
the contour of each member of the group can, from 
the circumstances of illumination then obtaining, 
be most distinctly inferred. 

Several comparatively small perfectly-formed 


VII.] 


A VOLCANIC REGION 


151 


craters are seen interspersed among the mountains, 
giving evidence of the truly volcanic character of 
the surrounding region, which, as before said, 
comprises in a comparatively limited space the 
most perfect and striking examples of nearly every 
class of lunar volcanic phenomena. 

We have endeavoured on Plate XXY. to give 
some idea of a landscape view of a small portion 
of this mountain range. 


CHAPTER VIII 


ON LUNAR CRATERS 

As we stated in our brief general description of the 
visible hemisphere of the moon, and as a cursory 
glance at our map and plates will have shown, the 
predominant features of the lunar surface are the 
circular or amphitheatrical formations that, by their 
number, and from their almost unnatural uniformity 
of design, induced the belief among early observers 
that they must have been of artificial origin. In 
proceeding now to examine the details of our 
subject with more minuteness than before, these 
annular formations claim the first share of our 
attention. 

By general acceptation the term “crater” has 
been used to represent nearly all the circular 
hollows that we observe upon the moon; and with¬ 
out doubt the word in its literal sense, as indicating 
a cup or circular cavity, is so far aptly applied. But 


chap, viiij THE TRUE VOLCANIC CHARACTER 153 

among geologists it has been employed in a more 
special sense to define the hollowing out that is 
found at the summit of some extinct, and the 
majority of active, volcanoes. In this special sense 
it may be used by the student of the lunar surface, 
though in some, and indeed in the majority of cases, 
the lunar crater differs materially in its form with 
respect to its surroundings from those on the earth ; 
for while, as we have said, the terrestrial crater is 
generally a hollow on a mountain top with its flat 
bottom high above the level of the surrounding 
country, those upon the moon have their lowest 
points depressed more or less deeply below the 
general surface of the moon, the external height 
being frequently only a half or one-third of the 
internal depth. Yet are the lunar craters truly 
volcanic; as Sir John Herschel has said, they offer 
the true volcanic character in its highest perfection. 
We have upon the earth some few instances in 
which the geological conditions which have deter¬ 
mined the surface-formation have been identical 
with those that have obtained upon the moon; and 
as a result we have some terrestrial volcanic districts 
that, could we view them under the same circum¬ 
stances, would be identical in character with what 
we see by telescopic aid upon our satellite. The 


154 ON LUNAR CRATERS [chap. 

most remarkable* case of this similarity is offered 
by a certain tract of the volcanic area about Naples, 
known from classic times as the Campi Phlegrwi , 
or burning fields, a name given to them in early 
days, either because they showed traces of ancient 
earth-fire, or because there were attached to the 
localities traditions concerning hot-springs and 
sulphurous exhalations, if not of actual fiery erup¬ 
tions. The resemblance of which we are speaking 
is here so close that Professor Phillips, in his work 
on Vesuvius, which by the way contains a historical 
description of the district in question, calls the 
moon a grand Phlegreian field. How closely the 
ancient craters of this famous spot resemble the 
generality of those upon the moon may be judged 
from Plates XV. and XVI., in which representa¬ 
tions of two areas, terrestrial and lunar, of the same 
extent, are exhibited side by side, the terrestrial 
region being the volcanic neighbourhood of Naples, 
and the lunar a portion of the surface about the 
crater Theophilus. 

In comparing these volcanic circles together, we 
are however brought face to face wdth a striking 
difference that exists between the lunar and terres¬ 
trial craters. This is the difference of magnitude. 
None of those Plutonian amphitheatres included in 



Plate XV.—Vesuvius, and neighbourhood of Naples. 


[To face page 154- 









VIII.] COMPARISON WITH VESUVIUS 155 

the terrestrial area depicted exceed a mile in dia¬ 
meter, and few larger volcanic vents than these are 
known upon the earth. Yet when we turn to the 
moon, and measure some of the larger craters there, 
we are astonished to find them ranging from an 
almost invisible minuteness to 74 miles in diameter. 
The same disproportion exists between the depths of 
the two classes of craters. To give an idea of relative 
dimensions, we would refer to our illustration of 
Copernicus # and its hundreds of comparatively 
minute surrounding craters. Our terrestrial Vesu¬ 
vius would be represented by one of these last, 
which upon the plate measures about the twentieth 
of an inch in diameter! And this disproportion 
strikes us the more forcibly when we consider that 
the lunar globe has an area only one-thirteenth of 
that of the earth. In view of this great apparent 
discrepancy it is not surprising that many should 
have been incredulous as to the true volcanic char¬ 
acter of the lunar mountains, and have preferred to 
designate them by some “non-committal” terms, as 
an American geologist (Professor Dana) has ex¬ 
pressed it. But there is a feature in the majority 
of the ring-mountains that, as we conceive, demon¬ 
strates completely the fact of volcanic force having 
* Plate VI. (facing p. 122). 


156 


ON LUNAR CRATERS 


[chap. 


been in full action, and that seems to stamp the 
volcanic character upon the crater-forms. This 
special feature is the central cone, so well known as 
a characteristic of terrestrial volcanoes, accepted as 
the result of the last expiring effort of the eruptive 
force, and formed by the deposit, immediately 
around the volcanic orifice, of matter which there 
was not force enough to project to a greater distance. 
Upon the moon we have the central cone in small 
craters comparable to those on the earth, and we 
have it in progressively larger examples, upon all 
scales, up to craters of 74 miles in diameter, as we 
have shown on p. 155. Where, then, can we draw 
the line ? Where can we say the parallel action to 
that which placed Yesuvius in or near the centre of 
the arc of Somma, or the cone figured in our 
sectional drawing of Vesuvius (Fig. 3) in the middle 
of its present crater—where can we say that the 
action in question ceased to manifest itself on the 
moon, seeing that there is no break in the continuity 
of the crater-and-cone system upon the moon any¬ 
where between craters of If miles and 74 miles in 
diameter? We have, it is true, many examples of 
coneless craters, but these are of all sizes, down to 
the smallest, and up to a magnitude that would 
almost seem to render untenable the ejective ex- 


Companion to Hell 

IViMUesDiimJ 


© 

Companion toThcbits 

2’4 Miles UiamH. 


& 

Small Crater. 

lNSrDE “WALTER* 



Companion to Parot 

H Miles DUarF 



H E RSCH EL 

]I Disusi! 



Godin. 

223files Discus? 



24- MilesDiaiEj 



Delambre. 
26 lEles Diani? 



Copernicus. 

46 Miles DiamT 



Tycho. 

50 Miles DiamT 




Eratosthnes. 

23 I'Eks Ttanv£ 



AGRI PPA. 

30 Miles Diana? 



2JMesIbam? 



Theophilus. 

GOEles Diaii? 




Diagram of Lunar Craters, forming a series ranging from If miles to 
78 miles in diameter, all containing central cones. 


[To face page 156. 




















VIII.] 


THE EJECTIVE FORCE 


157 


planation : of these we shall specially speak in turn, 
but for the present we will confine ourselves to the 
normal class of lunar craters, those that have central 
cones, and that are in all reasonable probability 
truly volcanic. 

And in the first place let us take a passing 
glance at the probable formative process of a ter¬ 
restrial volcano. Rejecting the hypothesis of Yon 
Buch, which geologists have on the whole found to 
be untenable, and which ascribes the formation of 
all mountains to the elevation of the earth’s crust 
by some thrusting power beneath, we are led to 
regard a volcano as a pyramid of ejected matter, 
thrown out of and around an orifice in the external 
solid shell of the earth by commotions engendered 
in its molten nucleus. What is the precise nature 
and source of the ejective force geologists have not 
perfectly agreed upon, but we may conceive that 
highly expanded vapour, in all probability steam, is 
its primary cause. The escaping aperture may have 
been a weak place since the foundations of the 
earth were laid, or it may have been formed by a 
local expansion of the nucleus in the act of cooling, 
upon the principle enunciated in our third chapter; 
or, again, the expansile vapour may have forced its 
own way through that point of the confining shell 


158 


ON LUNAR CRATERS 


[CHAP. 


that offered it the least resistance. The vent once 
formed, the building of the volcanic mountain com¬ 
menced by the out-belching of the lava, ashes, and 
scoriae, and the dispersion of these around the vent 
at distances depending upon the energy with which 
they were projected. As the action continued, the 
ejected matter would accumulate in the form of a 
mound, through the centre of which communication 
would be maintained with the source of the ejected 
materials and the seat of the explosive agency. The 
height to which the pile would rise must depend 
upon several conditions : upon the steady sustenance 
of the matter, and upon the form and weight of the 
component masses, which will determine the slope 
of the mountain's sides. Supposing the action to 
subside gradually, the tapering form will be con¬ 
tinued upwards by the comparatively gentle deposi¬ 
tion of material around the orifice, and a perfect cone 
will result of some such form as that represented 
in Fig. 16, which is the outline ascribed by Pro¬ 
fessor Phillips to Vesuvius in pre-historic, or even 
pre-traditional times, and which may be seen in its 
full integrity in the cases of Etna, Teneriffe, Fusi- 
Yama, the great volcanic mountain of Japan, and 
many others. The earliest recorded form of Vesu¬ 
vius is that of a truncated cone represented in Fig. 


VIII.] 


VESUVIUS 


159 


17, which shows its condition, according to Strabo, 
in the century preceding the Christian Era. Now 
this form may have been assumed under two con¬ 
ditions. If, as Phillips has surmised, the mountain 
originally had a peaked summit with but a small 
crater-orifice, at the point, then we must ascribe its 
decapitation to a subsequent eruption which in its 
violence carried away the upper portion, either 
suddenly, or through a comparatively slow process 
of grinding away or widening out of the sides of the 
orifice by the chafing or fluxing action of the out¬ 
going materials. But it is probable that the 
mountain never had the perfect summit indicated in 
our first outline. The violent outburst that caused 
the great crater-opening of our second figure may 
have been but one paroxysmal phase of the eruption 
that built the mountain : a sudden cessation of the 
eruptive force when at its greatest intensity, and 
when the orifice was at its widest, would leave 
matters in an opposite condition to that suggested 
as the result of a slow dying out of the action: 
instead of the peak we should have a wide crater- 
mouth. It is of small consequence for our present 
purpose whether the crater was contemporaneous 
with the primitive formation of the mountain, or 
whether it was formed centuries afterwards by the 


160 


ON LUNAR CRATERS 


[CHAP. 


blowing away of tlie mountain’s head; for upon the 
vast scale of geological time, intervals such as those 
between successive paroxysms of the same eruption, 
and those between successive eruptions, are scarcely 
to be discriminated, even though the first be days 
and the second centuries. We may remark that the 
widening of a crater by a subsequent and probably 
more powerful eruption than that which originally 
produced it is well established. We have only to 
glance at the sketch, Fig. 18, of the outline of 
Vesuvius as it appeared between the years a.d. 79 
and 1631 to see how the old crater was enlarged by 
the terrible Pompeian eruption of the first-mentioned 
year. Here we have a crater ground and blown 
away till its original diameter of a mile and three- 
quarters has been increased to nearly three miles. 
Scrope had no hesitation in expressing his convic¬ 
tion that the external rings, such as those of San- 
torin, St Jago, St Helena, the Cirque of Teneriffe, 
the Curral of Madeira, the cliff range that surrounds 
the island of Bourbon, and others of similar form 
and structure, however wide the area they enclose, 
are truly the “basal wrecks” of volcanic mountains 
that have been blown into the air each by some 
eruption of peculiar paroxysmal violence and per¬ 
sistence ; and that the circular or elliptical basins 



Fig. 16. 




Fig. 18. 



[To face page 1G0 























































- 










VIII.] 


THE INNER CONE 


161 


which they wholly or in part surround are in all 
cases true craters of eruption. 

When the violent outburst that produces a 
great crater in a volcanic mountain-top more or 
less completely subsides, the funnel or escaping 
orifice becomes choked with debris. Still the vent 
strives to keep itself open, and now and then gives 
out a small delivery of cindery matter, which, being 
piled around the vent, after the manner of its great 
prototype, forms the inner cone. This last may in 
its turn bear an open crater upon its summit, and 
a still smaller cone may form within it. As the 
action further dies away, the molten lava, no longer 
seething and boiling, and spirting forth with the 
rest of the ejected matter, wells upwards slowly, and 
cooling rapidly as it comes in contact with the atmos¬ 
phere, solidifies and forms a flat bottom or floor to 
the crater. 

It may happen that a subsequent eruption from 
the original vent will be comparable in violence to 
the original one, and then the inner cone assumes 
a magnitude that renders it the principal feature of 
the mountain, and reduces the old crater to a 
secondary object. This has been the case with 
Vesuvius. During the eruption of 1631 the great 
cone which we now call Vesuvius was thrown up, 

L 


162 


ON LUNAR CRATERS 


[CHAP. 


and the ancient crater now distinguished as Monte 
Somma became a subsidiary portion of the whole 
mountain. Then the appearance was that shown in 
Fig. 19, and which does not differ greatly from that 
presented in the present day. The summit of the 
Vesuvian cone, however, has been variously altered; 
it has been blown away, leaving a large crateral 
hollow, and it has rebuilt itself nearly upon its 
former model. 

When we transfer our attention to the volcanoes 
of the moon, we find ourselves not quite so well 
favoured with means for studying the process of 
their formation; for the sight of the building up of 
a volcanic mountain such as man has been per¬ 
mitted to behold upon the earth has not been 
allowed to an observer of the moon. The volcanic 
activity, enfeebled though it now be, of which we 
are witnesses from time to time on the earth, has 
altogether ceased upon our satellite, and left us 
only its effects as a clue to the means by which they 
were produced. If we in our time could have seen 
the actual throwing up of a lunar crater, our task of 
description would have been simple; as it is we are 
compelled to infer the constructive action from 
scrutiny of the finished structure. 

We can scarcely doubt that where a lunar crater 


VIII.] CRATERS: LUNAR AND TERRESTRIAL 163 


bears general resemblance to a terrestrial crater, 
the process of formation has been nearly the same 
in the one case as in the other. Where variations 
present themselves they may reasonably be ascribed 
to the difference of conditions pertaining to the two 
spheres. The greatest dissimilarity is in the point 
of dimensions; the projection of materials to 20 or 
more miles distance from a volcanic vent appears 
almost incredible, until we realise the full effect of 
the conditions which upon the moon are so favour¬ 
able to the dispersive action of an eruptive force. 
In the first place, the force of gravity upon our 
satellite is only one-sixth of that to which bodies 
are subject upon the earth. Secondly, by reason of 
the small magnitude of the moon and its propor¬ 
tionally much larger surface in ratio to its magni¬ 
tude, the rate at which it parted with its cosmical 
heat must have been much more rapid than in the 
case of the earth, especially when enhanced by the 
absence of the heat-conserving power of an atmos¬ 
phere of air or water vapour; and the disruptive 
and eruptive action and energy may be assumed to 
be greater in proportion to the more rapid rate of 
cooling; operating, too, as eruptive action would on 
matter so much reduced in weight as it is on the 
surface of the moon, we thus find in combination 


164 ON LUNAR CRATERS [chap. 

conditions most favourable to the display of vol¬ 
canic action in the highest degree of violence. 
Moreover, as the ejected material in .its passage 
from the centre of discharge had not to encounter 
any atmospheric resistance, it was left free to con¬ 
tinue the primary impulse of its ejection without 
other than gravitative diminution, and thus to de¬ 
posit itself at distances from its source vastly greater 
than those of which we have examples on the earth. 

We can of course only conjecture the source or 
nature of the moon’s volcanic force. If geologists 
have had difficulty in assigning an origin to the 
power that threw up our earthly volcanoes, into 
whose craters they can penetrate, whose processes 
they can watch, and whose material they can ana¬ 
lyse, how vastly more difficult must be the inquiry 
into the primary source of the power that has been 
at work upon the moon, which cannot be virtually 
approached by the eye within a distance of six or 
eight hundred miles, and the material of which we 
cannot handle to see if it be compacted by heat, or 
distended by vapours. Steam is the agent to which 
geologists have been accustomed to look for ex¬ 
planation of terrestrial volcanoes; the contact of 
water with the molten nucleus of our globe is 
accepted as a probable means whereby volcanic 


vnig CHARACTER OF THE EJECTIVE FORCE 165 

commotions are set up and ejective action is gener¬ 
ated. But we are debarred from referring to steam 
as an element of lunary geology, by reason of the 
absence of water from the lunar globe. We might 
suppose that a small proportion of water once ex¬ 
isted ; but a small proportion would not account 
for the immense display of volcanic action which 
the whole surface exhibits. If we admitted a 
Neptunian origin to the disturbances of the moon's 
crust, we should be compelled to suppose that water 
had existed nearly in as great quantity, area for 
area, there as upon our globe; but this we cannot 
reasonably do. 

Aqueous vapour being denied us, we must look 
in other directions for an ejective force. Of the 
nature of the lunar materials we can know nothing, 
and we might therefore assume anything; some 
have had recourse to the supposition of expansive 
vapours given off by some volatile component of the 
said material while in a state of fusion, or generated 
by chemical combinations. Professor Dana refers 
to sulphur as probably an important element in the 
moon's geology, suggesting this substance because 
of the part which it appears to play in the volcanic 
or igneous operations of our globe, and on account 
of its presence in cosmical meteors that have come 


166 


ON LUNAR CRATERS 


[CHAP. 


within range of our analysis. Any matter sublimated 
by heat in the substrata of the moon would be con¬ 
densed upon reaching the cold surrounding space, 
and would be deposited in a state of fine powder, 
or otherwise in a solid form. Maedler has at¬ 
tributed the highly reflective portions of some parts 
of the surface, such as the bright streams that 
radiate from some of the craters, Copernicus and 
Tycho for instance, to the vitrification of the surface 
matter by gaseous currents. But in suppositions 
like these we must remember that the probability 
of truth diminishes as the free ground for specula¬ 
tion widens. It does not appear clear how expan¬ 
sive vapours could have lain dormant till the moon 
assumed a solid crust, as all such would doubtless 
make their escape before any shell was formed, and 
at an epoch when there was ample facility for 
their expansion. 

While we are not insensible of the value of an 
expansive vapour explanation, if it could be based 
on anything beyond mere conjecture, we are dis¬ 
posed to attach greater weight to that afforded by 
the principle sketched in our third chapter, viz., of 
expansion upon solidification. We gave, as we think, 
ample proof that molten matter of volcanic nature, 
when about passing to the solid state, increases its 


VIII.] EXPANSION UPON SOLIDIFICATION 167 


bulk to a considerable degree, and we suggested 
that the lunar globe at one period of its history 
must have been, what our earth is now, a solid 
shell encompassing a molten nucleus; and further, 
that this last, in approaching its solid condition, 
expanded and burst open or rent its confining crust. 
At first sight it may seem that we are ascribing too 
great a degree of energy to the expansive force 
which molten substances exhibit in passing to the 
solid condition, seeing that in general such forces 
are slow and gradual in their action; but this 
anomaly disappears when we consider the vast bulk 
of the so expanding matter, and the comparatively 
small amount that in its expansion it had to displace. 
It is true that there are individual mountains on 
the moon covering many square miles of surface, 
that as much as a thousand cubic miles of material 
may have been thrown up at a single eruption; but 
what is this compared to the entire bulk of the 
moon itself? A grain of mustard-seed upon a 
globe three feet in diameter represents the scale of 
the loftiest of terrestrial mountains; a similar grain 
upon a globe one foot in diameter, would indicate 
the proportion of the largest upon the moon. A 
model of our satellite with the elevations to scale 
would show nothing more than a little roughness, 


168 


ON LUNAR CRATERS 


[chap. 


or superficial blistering. Turn for a moment to our 
map (Plate V.), upon which the shadows give in¬ 
formation as to the heights of the various irregu¬ 
larities, and suppose it to represent the actual size 
of some sphere whose surface has been broken up 
by reactions of some kind of the interior upon the 
exterior—suppose it to have been a globe of fragile 
material filled with some viscous substance, and 
that this has expanded, cracked its shell, oozed 
out in the process of solidification, and solidified : 
the irregularity of surface which the small sphere, 
roughened by the out-leaking matter, would present, 
would not be less than that exhibited in the map 
under notice. When we say that a lunar crater 
has a diameter of 30 miles, we raise astonishment 
that such a structure could result from an eruption 
by the expansive force of solidifying matter; but 
when we reflect that this diameter is less than the 
two-hundredth part of the circumference of the moon, 
we need have no difficulty in regarding the upheaval 
as the result of a force slight in comparison to the 
bulk of the material giving rise to it. We have 
upon the moon evidence of volcanic eruptions being 
the final result of most extensive dislocations of 
surface, such as could only be produced by some 
widely diffused uplifting force. We allude to the 


VIII.] WIDESPREAD UP-THRUSTING FORCE 169 


frequent occurrence of chains of craters lying in a 
nearly straight line, and of craters situated at the 
converging point of visible lines of surface disturb¬ 
ance. Our map will exhibit many examples of both 
cases. An examination of the upper portion (the 
southern hemisphere of the moon) will reveal abun¬ 
dant instances of the linear arrangement, three, four, 
five or even more crateral circles will be found to 
lie with their centres upon the same great-circle 
track, proving almost undoubtedly a connection 
between them so far as the original disturbing force 
which produced them is concerned. Again, in the 
craters Tycho (30), Copernicus (147), Kepler (146), 
and Proclus (162), we see instances of the situation 
of a volcanic outburst at an obvious focus of dis¬ 
turbance. These manifest an up-thrusting force 
covering a large sub-surface area, and escaping at 
the point of least resistance. Such an extent of 
action almost precludes the gaseous explanation, 
but it is compatible with the expansion on consoli¬ 
dation theory, since it is reasonable to suppose that 
in the process of consolidation the viscous nucleus 
would manifest its increase of bulk over considerable 
areas, disturbing the superimposed crust either in 
one long crack, out of the wider opening parts of 
which the expanded material would find its escape, 


170 


ON LUNAR CRATERS 


[CHAP. 


or “starring” it with numerous cracks, from the 
converging point of which the confined matter would 
be ejected in greatest abundance and, if ejected 
there with great energy and violence, would result 
in the formation of a volcanic crater. 

The actual process by which a lunar crater would 
be formed would differ from that pertaining to a 
terrestrial crater only to the extent of the different 
conditions of the two globes. We can scarcely 
accept Scrope’s term “basal wrecks” (of volcanic 
mountains that have had the summits blown away) 
as applicable to the craters of the moon, for the 
reason that the lunar globe does not offer us any 
instance of a mountain comparable in extent to the 
great craters and whose summit has not been blown 
away. Scrope’s definition implies a double, or 
divided process of formation : first the building up 
of a vast conical hill and then the decapitation and 
“evisceration” of it at some later period. There 
are grounds for this inferred double action among 
the terrestrial volcanoes, since both the perfect cone 
and its summitless counterpart are numerously ex¬ 
emplified. But upon the moon we have no perfect 
cone of great size, we have no exception whereby 
the rule can be proved. It is against probability, 
supposing every lunar crater to have once been a 



[To face 'page 170 . 


Plate XVII.—Normal Lunar Crater. 


































\ 








* 







t 












I 
































































*1 















\v 



Fig. 20. 



[To face page 171. 





VIII ] SECTION OF THE LUNAR SURFACE 171 


mountain, that in every case the mountain’s summit 
should have been blown away; and we are there¬ 
fore compelled to consider that the moon’s volcanic 
craters were formed by one continuous outburst, 
and that their “evisceration” was a part of the 
original formative process. We do not, however, 
include the central cone in this consideration : that 
may be reasonably ascribed to a secondary action 
or perhaps, better, to a weaker or modified phase of 
the original and only eruption. 

Under these circumstances we conceive the up¬ 
casting and excavating of a normal lunar crater to 
have been primarily caused by a local manifestation 
of the force of expansion upon solidification of the 
sub-surface matter of the moon, resulting in the 
creation of a mere “ star ” or crack in and through 
the outermost and solid crust. As we shall have 
to rely upon diagrams to explain the more com¬ 
plicated features, we give one of this elementary 
stage also as a commencement of the series; and 
Fig. 20 therefore represents a probable section of 
the lunar surface at a point which was subsequently 
the location of a crater. From the vent thus formed 
we conceive the pent-up matter to have found its 
escape, not necessarily at a single outburst, but in 
all probability in a paroxysmal manner, as volcanic 


172 


ON LUNAR CRATERS 


[CHAP. 


action manifests itself on our globe. The first out¬ 
flow of molten material would probably produce no 
more than a mere hill or tumescence as shown 
sectionally in Fig. 21; and if the ejective force were 
small this might increase to the magnitude of a 
mountain by an exudative process to be alluded to 
hereafter. But if the ejective force were violent, 
either at the moment of the first outburst or at any 
subsequent paroxysm, an action represented in Fig. 
22 would result: the unsupported edges or lips of 
the vent-hole would be blown and ground or fluxed 
away, and a funnel-formed cavity would be pro¬ 
duced, the ejected matter (so much of it as in 
falling was not caught by the funnel) being deposited 
around the hollow and forming an embryo circular 
mountain. The continuance of this action would 
be accompanied by an enlargement of the conical 
cavity or crater, not only by the outward rush of 
the violently discharged material, but also by the 
“ sweating ” or grinding action of such of it as in 
descending fell within the hollow. And at the 
same time that the crater enlarged the rampart 
would extend its circumference, for it would be 
formed of such material as did not fall back again 
into the crater. Upon this view of the crater¬ 
forming process we base the sketch, Fig. 23, of the 



Fig. 22. 



Fig. 23. 


[To face page 172. 
















n. . 




' 













VIII.] 


THE CENTRAL CONE 


173 


probable section of a lunar crater at one period of 
its development. 

So long as each succeeding paroxysm was 
greater than its predecessor, this excavating of the 
hollow and widening of its mouth and mound 
would be extended. But when a weaker outburst 
came, or when the energy of the last eruption died 
away, a process of slow piling up of matter close 
around the vent would ensue. It is obvious that 
when the ejective force could no longer exert itself 
to a great distance it must merely have lifted its 
burden to the relieving vent and dropped it in the 
immediate neighbourhood. Even if the force were 
considerable, the effect, so long as it was insufficient 
to throw the ejecta beyond the rim of the crater, 
would be to pile material in the lowermost part of 
the cavity; for what was not cast over the edge 
would roll or flow down the inner slope and accumu¬ 
late at the bottom. And as the eruption died away, 
it would add little by little to the heap, each ex¬ 
piring effort leaving the out-giving matter nearer the 
orifice, and thus building up the central cone that is 
so conspicuous a feature in terrestrial volcanoes, and 
which is also a marked one in a very large propor¬ 
tion of the craters of the moon. This formation of 
the cone is pictorially described by Fig. 24. 


174 


ON LUNAR CRATERS 


[chap. 


In the volcanoes of the earth we observe another 
action either concurrent with or immediately sub¬ 
sequent to the erection or formation of the cone : 
this is the outflow or the welling forth of fluid lava, 
which in cooling forms the well-known plateau. 
We have this feature copiously represented upon 
the moon, and it is presumable that it has in general 
been produced in a manner analogous to its counter¬ 
parts upon the earth. We may conceive that the 
fluid matter was either spirted forth with the solid 
or semi-solid constituents of the cone, in which case 
it would drain down and fill the bottom of the 
crater; or we may suppose that it issued from the 
summit of the cone and ran down its sides, or that, 
as we see upon the earth, it found its escape before 
reaching the apex, by forcing its way through the 
basal parts. These actions are indicated hypo¬ 
thetically for the moon in Fig. 25; and the parallel 
phenomena for the earth are shown by the actual 
case (represented in Fig. 26 and on Plate I.) of 
Vesuvius as it was seen by one of the authors in 
1864, when the principal cone was vomiting forth 
ashes, stones, and red-hot lava, while a vent at the 
side emitted very fluid lava which was settling down 
and forming the plateau. 

Although we cannot, obviously, see upon the 





Fig. 24. 



Fig. 25. 


[To face page 174 . 




































Fig. 26. 



Fig. 27. 


[To face page 174 . 




















































VIII.] A RISING LAKE OF LAVA 175 

moon evidence of a cone actually overtopped by the 
rising lake of lava, yet it is not unreasonable to 
suppose that such a condition of things actually 
occurred in many of those instances in which we 
observe craters without central cones, but with 
plateaux so smooth as to indicate previous fluidity 
or viscosity. From the state of things exhibited in 
Fig. 25 the transition to that shown in Fig. 27 is 
easily, and to our view reasonably, conceivable. 
We are in a manner led up to this idea by a review 
of the various heights of central cones above their 
surrounding plateaux. For instance, in such ex¬ 
amples as Tycho or Theophilus, we have cones high 
above the lava floor; in Copernicus, Arzachael, and 
Alphons they are comparatively lower; the lava in 
these and some other craters does not appear to 
have risen so high ; while in Aristotle and Eudoxus 
among others, we have only traces of cones, and it 
is supposable that in these cases the lava rose so 
high as nearly to overtop the central cones. Why 
should it not have risen so far as to overtop and 
therefore conceal some cones entirely? We offer 
this as at least a feasible explanation of some cone¬ 
less craters: it is not necessary to suppose that it 
applies to all such, however: there may have been 
many craters, the formation of which ceased so 


176 


ON LUNAR CRATERS 


[chap. 


abruptly that no cone was produced, though the 
welling forth of lava occurred from the vent, which 
may have been left fully open, as in Fig. 28, or so 
far choked as to stay the egress of solid ejecta and 
yet allow the fluid material to ooze upwards through 
it, and so form a lake of molten lava which on con¬ 
solidation became the plateau. As most of the ex¬ 
amples of coneless craters exhibit on careful exami¬ 
nation minute craters on the surface of the otherwise 
smooth plateaux, we may suppose that such minute 
craters are evidences of the upflow of lava which 
resulted in the plateaux. 

We have strong evidence in support of this up¬ 
flow of lava offered by the case of the crater War- 
gentin (No. 29), situated near the south-east border 
of the disc, and of which we give a special plate. 
(Plate XVIII.) It appears to be really a crater in 
which the lava has risen almost to the point of over¬ 
flowing, for the plateau is nearly level with the edge 
of the rampart. This edge appears to have been 
higher on one side than the other, for on the portion 
nearest the centre of the visible disc we may, under 
favourable circumstances, detect a segment of the 
basin’s brim rising above the smooth plateau as in¬ 
dicated in our illustration. Upon the opposite side 
there is no such feature visible, the plateau forms a 



Plate XVIII.—Wargentin. 


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[To face page 176 . 







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Fig. 29. 


[To face page 177 . 































VIII.] 


WARGENTIN 


177 


sharp table-like edge. It is just possible that an 
actual overflow of lava took place at this part of 
the crater, but from the unfavourable situation of 
this remarkable object it is impossible to decide the 
point by observation. There is no other crater upon 
the visible hemisphere of the moon that exhibits 
this filled-up condition; but, unique as it is, it is 
sufficient to justify our conclusion that the plateau¬ 
forming action upon the moon has been a flowing- 
up of fluid matter from below subsequent to the 
formation of the crater-rampart, and not, as a casual 
glance at the great smooth-bottom craters might 
lead us to suspect, a result of some sort of diluvial 
deposit which has filled hollows and cavities and so 
brought up an even surface. The elevated basin of 
Wargentin could not have been filled thus while the 
surrounding craters with ramparts equally or less 
high remained empty; its contained matter must 
have been supplied from within, we must conjecture 
by the upflow of lava from the orifice which gave 
forth the material to form the crateral rampart in 
the first instance. We are free to conjecture that 
at some period of this table-mountain’s formation it 
was a crater with a central cone, and that the rising 
lava over-topped this last feature in the manner 
shown by Fig. 29, 


178 ON LUNAR CRATERS [chap. 

The question occurs whether other craters may 
not have been similarly filled and have emptied 
themselves by the bursting of the wall under the 
pressure of the accumulated lake of lava within. 
We know that this breaching is a common pheno¬ 
menon in the volcanoes of our globe; the district 
of Auvergne furnishing us with many examples ; 
and there are some suspicious instances upon the 
moon. Copernicus exhibits signs of such disruption, 
as also does the smaller crater intruding upon the 
great circle of Gassendi. (See Plate VII.) But 
the existence of such discharging breaches implies 
the out-pouring of a body of fluid or semi-fluid 
material, comparable in cubical content to the 
capacity of the crater, and of this we ought to see 
traces or evidence in the form of a bulky or exten¬ 
sive lava stream issuing from the breach. But 
although there are faint indications of once viscous 
material lying in the direction that escaping fluid 
would take, we do not find anything of the extent 
that we should expect from the mass of matter that 
would constitute a craterfull. It is true that if the 
escaping fluid had been very limpid it might have 
spread over a large area and have formed a 
stratum too thin to be detected. Such a degree 
of limpidity as would be required tb fulfil this 


VIII.] GROUPS OF CONICAL HILLS 179 

condition we are hardly, however, justified in 
assuming. 

To return to the subject of central cones. Al¬ 
though there are cases in which the simple condi¬ 
tion of a single cone exists, yet in the majority we 
see that the cone-forming process has been divided 
or interrupted, the consequence being the produc¬ 
tion of a group of conical hills instead of a single 
one. Copernicus offers an example of this char¬ 
acter, six, some observers say seven, separate points 
of light, indicating as many peaks tipped with sun¬ 
shine, having been seen when the greater part of the 
crater has been buried in shadow. Eratosthenes, 
Bulialdus, Maurolicus, Petavius, Langreen, and 
Gassendi, are a few among many instances of craters 
possessing more than a central single cone. This 
multiplication of peaks upon the moon doubtless 
arose from similar causes to those which produce 
the same feature in terrestrial volcanoes. Our 
sketch of Vesuvius in 1864 (Plate I. and Fig. 26) 
shows the double cone and the probable source of 
the secondary one in the diverted channel of the out- 
coming material. A very slight interruption in the 
first instance would suffice to divert the stream and 
form another centre of action, or a choking of the 
original vent would compel the issuing matter to 


180 


ON LUNAR CRATERS 


[CHAP. 


find a less resisting thoroughfare into open space, 
and the process of cone-building would be continued 
from the new orifice, perhaps to be again inter¬ 
rupted after a time and again driven in another 
direction. In this manner, by repeated arrests and 
diversions of the ejecta, cone has grown upon the 
side of cone, till, ere the force has entirely spent 
itself, a cluster of peaks has been produced. It 
may have been that this action has taken place after 
the formation of the plateau, in the manner indicated 
by Fig. 30; a spasmodic outburst of comparatively 
slight violence having sought relief from the original 
vent, and the flowing matter, finding the one orifice 
not sufficiently open to let it pass, having forced 
other exit through the plateau. 

In frequent instances we observe the state of 
things represented in Fig. 31, in which the plateau 
is studded with few or many small craters. This is 
the case with Plato, with Arzaehael, Hipparchus, 
Clavius (which contains about 15 small internal 
craters), and many others. It is probable that 
these subsidiary craters were produced by an after¬ 
action like that which has produced duplicated 
cones, but in which the secondary eruption has been 
of somewhat violent character, for it may almost be 
regarded as an axiom that violent eruptions ex- 



Fig. 30. 



Fig. 31. 


[To face page 180 . 





























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VIII.] CRATERS WITHIN CRATERS 181 

cavate craters and weak ones pile up cones. In the 
cases referred to it seems reasonable to suppose 
that the main vent has been the channel for an up¬ 
cast of material, but that at some depth below the 
surface this material met with some obstruction or 
cause of diversion, and that it took a course which 
brought it out far away from the cone upon the 
floor of the plateau. It might even be carried so 
far as to be upon the rampart, and it is no un¬ 
common thing to see small craters in such a situa¬ 
tion, though when they appear at such a distance 
from the primary vent, it seems more reasonable to 
suppose that they do not belong to it, but have 
arisen from a subsequent and an independent 
action. 

We find scarcely an instance of a small crater 
occurring just in the centre of a large one, or taking 
the place of the cone. This is a curious circum¬ 
stance. Whenever we have any central feature in 
a great crater that feature is a cona The tendency 
of this fact is to prove that cones were produced by 
very weak efforts of-this expiring force, for had 
there been any strength in the last paroxysm it is 
presumable that it would have blown out and left 
a crater. No very violent eruptions have therefore 
taken place from the vents that were connected with 


182 


ON LUNAR CRATERS 


[chap. 


the great craters of the moon, nothing more power¬ 
ful than could produce a cone of exudation or a 
cinder-heap. And with regard to cones, it is note¬ 
worthy that whether they be single or multiple, 
they never rise so high as the circular ramparts of 
their respective craters. This supports the inferred 
connection between the crater origin and the cone 
origin, for supposing the two to have been indepen¬ 
dent, a supposition untenable in view of the uni¬ 
versality of the central position of the cone, it is 
scarcely conceivable that the mountains should have 
always been located within ramparts higher than 
themselves. The less height argues less power in 
the upcasting agency, and the diminished force may 
well be considered as that which would almost of 
necessity precede the expiration of the eruption. 

Occasionally a crater is met with that has a 
double rampart, and the concentricity suggests that 
there have been two eruptions from the same vent: 
one powerful, which formed the exterior circle, and 
a second rather less powerful which has formed the 
interior circle. It is not, however, evident that this 
duplication of the ring has always been due to a 
double eruption. In many cases there is duplica¬ 
tion of only a portion: a terrace exhibits itself 
around a part of the circular range, sometimes upon 


VIII.] 


LANDSLIPS 


183 


the outside and sometimes upon the inside. These 
terraces are not likely to have been formed by any 
freak of the eruption, and we are led to ascribe 
them in general to landslip phenomena. When, in 
the course of a volcano's formation, the piling-up of 
material about the vent has continued till the lower 
portions have been unable to support the upper, or 
when from any cause the material composing the 
pile has lost its cohesiveness, the natural conse¬ 
quence has been a breaking away of a portion of 
the structure and its precipitation down the inclined 
sides of the crater. Vast segments of many of the 
lunar mountain-rings appear to have been thus dis¬ 
lodged from their original sites and cast down the 
flanks to form crescent ranges of volcanic rocks 
either within or without the circle. Nearly every 
one of our plates contains craters exhibiting this 
feature in more or less extensive degree. Some¬ 
times the separated portion has been very small in 
proportion to the circumference of the crater : Plato 
is an instance in which a comparatively small mass 
has been detached. In other cases very large 
segments have slid down and lie in segmental 
masses on the plateaux or form terraces around the 
rampart. Aristarchus, Triesnecker, and Copernicus 
exhibit this larger extent of dislocation. 


184 


ON LUNAR CRATERS 


[CHAP. 


It is possible that these landslips occurred long 
after the formation of the craters that have been 
subject to them. They are probably attributable 
to recent disintegration of the lunar rocks, and we 
have a powerful cause for this in the alternations of 
temperature to which the lunar crust is exposed. 
We shall have occasion to revert to this subject by- 
and-by ; at present it must suffice to point out that 
the extremes of cold and heat, between which the 
lunar soil varies, are, with reasonable probability, 
assumed to be on the one hand the temperature of 
space (which is supposed to be between 200° and 
250° below zero), and on the other hand, a degree of 
heat equal to about twice that of boiling water. A 
range of at least 500° must work great changes in 
such heterogeneous materials as we may conjecture 
those of the lunar crust to be, by the alternate con¬ 
tractions and expansions which it must engender, 
and which must tend to enlarge existing fissures 
and create new ones, to grind contiguous surfaces 
and to dislodge unstable masses. This cause of 
change, it is to be remarked, is one which is still 
exerting itself. 

In a few cases we have an entirely opposite 
interruption of the uniformity of a crater’s contour. 
Instead of the breaking away of the ring in seg- 


VIII.] 


RUTTED BANKS 


185 


ments, we see the entire circuit marked with deep 
ruts that run down the flanks in a radial direction, 
giving us evidence of a downward streaming of 
semi-fluid matter, instead of a disruption of solid 
masses. We cannot doubt that these ruts have 
been formed by lava currents, and they indicate a 
condition of ejected material different from that 
which existed in the cases where the landslip char¬ 
acter is found. In these last the ejecta appear to 
have been in the form of masses of solidified or 
rapidly solidifying matter, which remained where 
deposited for a time and then gave way from over¬ 
loading or loss of cohesiveness, whereas the sub¬ 
stances thrown out in the case of the rutted banks 
were probably mixed solid and fluid, the former 
remaining upon the flanks while the latter trickled 
away. Nothing so well represents, upon a small 
scale, this radial channelling as a heap of wetted 
sand left for a while for the water to drain off from 
it. The solid grains in such a heap sustain its 
general mass-form, but the liquid in passing away 
cuts the surface into fissures running from the sum¬ 
mit to the base, and forms it into a model of a 
volcanic mountain with every feature of peak, crag, 
and chasm reproduced. This similarity of effect 
leads us to suspect a parallelism of cause, and thus 


186 


ON LUNAR CRATERS 


[chap. 


to the inference that the material which originally 
formed such a crater-mountain as Aristillus (which 
is a most prominent example of this rutted char¬ 
acter, and appears in Plate IX., side by side with a 
crater that has its banks segmentally broken), must 
have been of the compound nature indicated; and 
that an action analogous to that which ruts a damp 
sand-heap, rutted also the banks of the lunar 
crater. 

Before passing from the subject of craters it 
behoves us to say a few words upon the curious 
manner in which these formations are complicated 
by intermingling and superposition. Yet, upon this 
point, we may be brief, for in the way of description 
our plates speak more forcibly than is possible by 
words. In particular we would refer to Plate X., 
which represents the conspicuous group of craters 
of which the three largest members have been respec¬ 
tively named Theophilus, Cyrillus, and Catharina. 
But the area included in this plate is by no means 
an extraordinary one; there are regions about 
Tycho wherein the craters so crowd and elbow each 
other that, in their intricate combinations, they 
almost defy accurate depiction. Our map and 
Plate XIII. will serve to give some idea of them. 
This intermingling of craters obviously shows that 


VIII.] 


OVERLAPPING CRATERS 


187 


all the lunar volcanoes were not simultaneously pro¬ 
duced, but that after one had been formed, an erup¬ 
tion occurred in its immediate neighbourhood and 
blew a portion of it away; or it may have been that 
the same deep-seated vent at different times gave 
forth discharges of material the courses of which 
were more or less diverted on their way to the 
surface. 

We have before alluded to the frequent occur¬ 
rence of lines of craters upon the moon. In these 
lines the overlapping is frequently visible; it is seen 
in Plate X. before referred to, where the ring- 
mountains are linked into a chain slightly curved, 
and upon the map, Plate V., the nearly central 
craters Ptolemy and Alphons, the latter of which 
overlaps the former, are seen to form part of a line 
of craters marking a connection of primary dis¬ 
turbance. An extensive crack suggests itself as a 
favourable cause for the production of this over¬ 
laying of craters, for it would serve as a sort of 
“line of fire” from various points at which erup¬ 
tions would burst forth, sometimes weak or far 
apart, when the result would be lines of isolated 
craters, and sometimes near together, or powerful, 
when the consequence would be the intrusion of one 
upon the other, and the perfect production of the 


188 


ON LUNAR CRATERS 


[chap. 


latest formed at the expense or to the detriment of 
those that had been formed previously. The linear 
grouping of volcanoes upon the earth long ago 
struck observant minds. The fable of the Typhon 
lying under Sicily and the Phlegreian fields and 
disturbing the earth by its writhings, is a mytho¬ 
logical attempt to explain the particular case in that 
region. 

The capricious manner in which these intrusions 
occur is very curious. Very commonly a small 
crater appears upon the very rampart of a greater 
one, and a more diminutive one still will appear 
upon the rampart of the parasite. Stoefiler presents 
us with one example of this character, Hipparchus 
with another, Maurolycus with a third, and these 
are but a few cases of many. Here and there we 
observe several craters ranged in a line with their 
rims in one direction all perfect, and the whole 
appearing like a row of coins that have fallen from 
a heap. There is an example near to Tycho which 
we reproduce in Plate XIX. In this case one is 
led to conjecture that the ejective agency, after 
exerting itself in one spot, travelled onward and 
renewed itself for a time; that it ceased after form¬ 
ing crater number two, and again journeyed forward 
in the same line, recommencing action some miles 



Plate XIX.—Overlapping Craters. 


to s o 

MILES 


20 30 40 SO 
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Scale 


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[To face page] |188i 




VIII.] LINES OF CRATERS 189 

further, and again subsiding; yet again pushing 
forward and repeating its outburst, till it produced 
the fourth crater, when its power became expended. 
In each of these successive eruptions the centre of 
discharge has been just outside the crater last 
formed; and the close connection of the members 
of the group, together with the fact of their nearly 
similar size, appears to indicate a community of 
origin. For it seems feasible that as a general rule 
the size of a crater may be taken as a measure of 
the depth of force that gave rise to the eruption 
producing it. This may not be true for particular 
cases, but it will hold where a great number are 
collectively considered ; for if we assume the exist¬ 
ence of an average disturbing force, it is apparently 
clear that it will manifest itself in disturbing greater 
or less surface-areas in proportion as it acts from 
greater or less depths. Or, mutatis mutandis , if we 
assume an uniform depth for the source of action, 
the greater or less surface disturbance will be a 
measure of greater or less eruptive intensity. 

Perhaps the most remarkable case of a vast 
number of craters, which, from their uniform dimen¬ 
sions, suggest the idea of community of source- 
power or source-depth, is that offered by the region 
surrounding Copernicus, which, as will be seen by 


190 


ON LUNAR CRATERS 


[chap. 


our plate of that object, is a vast Phlegreian field of 
diminutive craters. So countless are the minute 
craters that a high magnifying power brings into 
view when atmospheric circumstances are favour¬ 
able, and so closely are they crowded together, that 
the resulting appearance suggests the idea of froth, 
and we should be disposed to christen this the 
“frothy region” of the moon, did not a danger 
exist in the tendency to connect a name with a 
cause. The craters that are here so abundant are 
doubtless the remains of true volcanoes analogous 
to the parasitical cones that are to be found on 
several terrestrial mountains, and not such accidental 
formations as the Hornitos described by Humboldt 
as abounding in the neighbourhood of the Mexican 
volcano, Jurillo, but which the traveller did not 
consider to be true cones of eruption. # Although 
upon our plate, and in comparison with the great 
crater that is its chief feature, these countless 
hollows appear so small as at first sight to appear 
insignificant, we must remember that the minutest 
of them must be grand objects, each probably equal 
in dimensions to Vesuvius. For since, as we have 
shown in an early chapter, the smallest discernible 
telescopic object must subtend an angle to our eye 

* Cosmos, Bohn’s Edition, vol. v., p. 322. 


VIII.] 


A FIFTY-MILE CRATER 


191 


of about a second, and since this angle extended to 
the moon represents a mile of its surface, it follows 
that these tiny specks of shadow that besprinkle our 
picture, are in the reality craters of a mile diameter. 
This comparison may help the conception of the 
stupendous magnitude of the moon’s volcanic 
features; for it is a conception most difficult to 
realise. It is hard to bring the mind to grasp the 
fact that that hollow of Copernicus is fifty miles in 
diameter. We read of an army having encamped 
in the once peaceful crater of Vesuvius, and of one 
of the extinct volcanoes of the Campi Phlegrcei being 
used as a hunting preserve by an Italian king. 
These facts give an idea of vastness to those who 
have not the good fortune to see the actual dimen¬ 
sions of a volcanic orifice themselves. But it is 
almost impossible to conjure up a vision of what 
that fifty-mile crater would look like upon the moon 
itself; and for want of a terrestrial object as a 
standard of comparison, our picture, and even the 
telescopic view of the moon itself, fails to render the 
imagination any help. We may try to realise the 
yastness by considering that one of our average 
English counties could be contained within its 
ramparts, or by conceiving a mountainous amphi¬ 
theatre whose opposite sides are as far apart 


192 


ON LUNAR CRATERS 


[CHAP. VIII. 


as the cathedrals of London and Canterbury, 
but even these comparisons leave us unimpressed 
with the true majesty which the object would 
present to a spectator upon the surface of our 
satellite. 


I 










1 




CHAPTER IX 


ON THE GREAT RING-FORMATIONS NOT MANIFESTLY 
VOLCANIC 

In our previous chapter we have given a reason for 
regarding as true volcanic craters all those circular 
formations, of whatever size, that exhibit that dis¬ 
tinctive feature the central cone. Between the 
smallest crater with a cone that we can detect 
under the best telescopic conditions, namely, the 
companion to Hell, If mile diameter, and the great 
one called Petavius, 78 miles in diameter, we find 
no break in the continuity of the crater-cum-cone 
system that would justify us in saying that on the 
one side the volcanic or eruptive cause ceased, and 
on the other side some other causative action began. 
But there are numerous circular formations that 
surpass the magnitude of Petavius and its peers, 
but that have no circular cone, and are, therefore, 
not so manifestly volcanic as those which possess 


194 THE GREAT RING-FORMATIONS [chap. 

this feature. Our map will show many striking ex¬ 
amples of this class at a glance. We may in par¬ 
ticular refer inter alia to Ptolemy near the centre of 
the moon, to Grimaldi (No. 125), Schickard (No 28), 
Schiller (No. 24), and Clavius (No. 13), all of which 
exceed 100 miles in diameter. Even the great Mare 
Crisium , nearly 300 miles in diameter, appears to 
be a formation not distinct from those which we 
have just named. These present little of the generic 
crater character in their appearance; and they have 
been distinguished therefrom by the name of Walled 
or Ramparted Plains . Their actual origin is beyond 
our explanation, and in attempting to account for 
them we must perforce allow considerable freedom 
to conjecture. They certainly, as Hooke suggested, 
present a “ broken bubble’’-like aspect; but one 
cannot reasonably imagine the existence of any form 
of mineral matter that would sustain itself in bubble 
form over areas of many hundreds of square miles. 
And if it were reasonable to suppose the great rings 
to be the foundations of such vast volcanic domes, 
we must conclude these to have broken when they 
could no longer sustain themselves, and in that case 
the surface beneath should be strewed with debris, 
of which, however, we can find no trace. Moreover, 
we might fairly expect that some of the smaller 


IX.] 


CAUSES OF CIRCULARITY 


195 


domes would have remained standing: we need 
hardly say that nothing of the kind exists. 

The true circularity of these objects appears at 
first view a remarkable feature. But it ceases to be 
so if we suppose them to have been produced by 
some very concentrated sublunar force of an up¬ 
heaving nature, and if only we admit the homo¬ 
geneity of the moon’s crust. For if the crust be 
homogeneous, then any upheaving force, deeply 
seated beneath it, will exert itself with equal effects 
at equal distances from the source: the lines of equal 
effect will obviously be radii of a sphere with the 
source of the disturbance for its centre, and they 
will meet a surface over the source in a circle. This 
will be evident from Fig. 82, in which a force is 
supposed to act at F below the surface s s s s. The 
matter composing s s being homogeneous, the action 
of F will be equal at equal distances in all direc¬ 
tions. The lines of equal force, F 'f F 'f will be of 
equal length, and they will form, so to speak, radii 
of a sphere of force. This sphere is cut by the plane 
at s s s s, and as the intersection necessarily takes 
place everywhere at the extremity of these radii, the 
figure of intersection is demonstrably a circle (shown 
in perspective at an ellipse in the figure). Thus we 
see that an intense but extremely confined explosion, 


196 


THE GREAT RING-FORMATIONS [chap. 


for instance, beneath the moon’s crust must disturb 
a circular area of its surface, if the intervening 
material be homogeneous. If this be not homo¬ 
geneous there would be, where it offered less than 
the average resistance to the disturbance, an out¬ 
ward distortion of the circle; and an opposite inter¬ 
ruption to circularity if it offers more than the 
average resistance. This assumed homogeneity 
may possibly be the explanation of the general cir¬ 
cularity of the lunar surface features, small and 
great. 

We confess to a difficulty in accounting for such 
a very local generation of a deep-seated force; and, 
granting its occurrence, we are unprepared with a 
satisfactory theory to explain the resultant effect of 
such a force in producing a raised ring at the limit 
of the circular disturbance. We may, indeed, sup¬ 
pose that a vast circular cake or conical frustra 
would be temporarily upraised as in Fig. 33, and 
that upon its subsidence a certain extrusion of sub¬ 
surface matter would occur around the line or zone 
of rupture as in Fig. 34. This supposition, however, 
implies such a peculiarly cohesive condition of the 
matter of the uplifted cake, that it is doubtful 
whether it can be considered tenable. We should 
expect any ordinary form of rocky matter subjected 


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of the intervening masses, c c. Wedges forced upwards by horizontal compression. , 
e f. Neutral plane or pivot axis, above and below which the directions of the tearing 
strain and horizontal compression are severally indicated by the smaller arrows ; the 
larger arrows beneath represent the direction of the primary expansive force. 

































IX.] 


A RING OF DISRUPTION 


197 


to such an upheaval to be fractured and distorted, 
especially when the original disturbing force is 
greater in the centre than at the edge, as, according 
to the above hypothesis, it would be; and in sub¬ 
siding, the rocky plateau would thus retain some 
traces of its disturbance; but in the circular areas 
upon the moon there is nothing to indicate that 
they have been subjected to such dislocations. 

Mr Scrope in his work on volcanoes has given 
a hypothetical section of a portion of the earth’s 
crust, which presents a bulging or tumescent surface 
in some measure resembling the effect which such 
a cause as we have been considering would produce. 
We give a slightly modified version of his sketch in 
Fig. 35, showing what would be the probable pheno¬ 
mena attending such an upheaval as regards the 
behaviour of the disturbed portion of the crust, and 
also that of the lava or semi-fluid matter beneath : 
and, as will be seen by the sketch, a possible phase 
of the phenomena is the production of an elevated 
ridge or rampart at the points of disruption c c; and 
where there is a ring of disruption, as by our hy¬ 
pothesis there would be, the ridge or rampart c c 
would be a circle. In this drawing we see the 
cracking and distortion to which the elevated area 
would be subjected, but of which, as previously re- 


198 THE GREAT RING-FORMATIONS [chap. 

marked, the circular areas of the moon present no 
trace of residual appearance. 

Those who have offered other explanations of 
these vast ring-formed mountain ranges, have been 
no more happy in their conjectures. M. Eozet, who 
communicated a paper on selenology to the French 
Academy in 1846, put forth the following theory. 
He argued that during the formation of the solid 
scoriaceous pelicules of the moon, circular or tour- 
billonic movements were set up; and these, by 
throwing the scoriae from the centre to the circum¬ 
ference, caused an accumulation thereof at the limit 
of the circulation. He considered that this pheno¬ 
menon continued during the whole process of solidi¬ 
fication, but that the amplitude of the whirlpool 
diminished with the decreasing fluidity of the sur¬ 
face material. Further, he suggested that when 
many vortices were formed, and the distances of 
their centres, taken two and two, were less than the 
sums of their radii, there resulted close spaces ter¬ 
minated by arcs of circles; and when for any two 
centres the distance was greater than the sum of 
their radii of action, two separate and complete 
rings were formed. We have only to remark on 
this, that we are at a loss to account for the origina¬ 
tion of such vorticose movements, and M. Eozet is 


IX.] 


THE EBULLITION THEORY 


199 


silent on the point. If the great circles are to be 
referred to an original sea of molten matter, it 
appears to us more feasible to consider that wher¬ 
ever we see one of them there has been, at the 
centre of the ring, a great outflow of lava that has 
flooded the surrounding surface. Then, if from any 
cause, and it is not difficult to assign one, the out¬ 
flow became intermittent, or spasmodic, or subject 
to sudden impulses, concentric waves would be pro¬ 
pagated over the pool and would throw up the scoria 
or the solidifying lava in a circular bank at the limit 
of the fluid area. 

This hypothesis does not differ greatly from the 
ebullition theory proposed by Professor Dana, the 
American geologist, to explain these formations. 
He considered that the lunar ring-mountains were 
formed by an action analogous to that which is ex¬ 
emplified on the earth in the crater of Kilauea, in 
the Hawaiian islands. This crater is a large open 
pit exceeding three miles in its longer diameter, and 
nearly a thousand feet deep. It has clear bluff walls 
round a greater part of its circuit, with an inner 
ledge or plain at their base, raised 340 feet above 
the bottom. This bottom is a plain of solid lavas, 
entirely open to-day, which may be traversed with 
safety (we are quoting Professor Dana’s own state- 


200 THE GREAT RING-FORMATIONS [chap. 

ment written in 1846, and therefore not correctly 
applying to the present time): over it there are 
pools of boiling lava in active ebullition, and one is 
more than a thousand feet in diameter. There are 
also cones at times, from a few yards to two or 
three thousand feet in diameter, and varying greatly 
in angle of inclination. The largest of these cones 
have a circular pit or crater at the summit. The 
great pit itself is oblong, owing to its situation on a 
fissure, but the lakes upon its bottom are round, 
and in them, says Professor Dana, “the circular or 
slightly elliptical form of the moon’s craters is ex¬ 
emplified to perfection.” 

Now Dana refers this great pit crater and its 
contained lava-lakes to “ the fact that the action at 
Kilauea is simply 'boiling , owing to the extreme 
fluidity of the lavas. The gases or vapours which 
produce the state of active ebullition escape freely 
in small bubbles, with little commotion, like jets 
over boiling water; while at Vesuvius and other like 
cones they collect in immense bubbles before they 
accumulate force enough to make their way through ; 
and consequently the lavas in the latter case are 
ejected with so much violence that they rise to a 
height often of many thousand feet and fall around 
in cinders. This action builds up the pointed moun- 


IX.] 


PROFESSOR DANA’S HYPOTHESIS 


201 


tain, while the simple boiling of Kilauea makes no 
cinders and no cinder cones.” 

Professor Dana continues, “If the fluidity of 
lavas, then, is sufficient for this active ebullition, we 
may have boiling going on over an area of an in¬ 
definite extent; for the size of a boiling lake can 
have no limits except such as may arise from a 
deficiency of heat. The size of the lunar craters is 
therefore no mystery. Neither is their circular 
form difficult of explanation; for a boiling pool 
necessarily, by its own action, extends itself circu¬ 
larly around its centre. The combination of many 
circles, and the large sea-like areas, are as readily 
understood.” # 

In justice to Professor Dana, it should be stated 
that he included in this theory of formation all 
lunar craters, even those of small size and possess¬ 
ing central cones; and he put forth his views in 
opposition to the eruptive theory which we have set 
forth, and which was briefly given to the world 
more than twenty-five years ago. As regards the 
smallest craters with cones, we believe few geolo¬ 
gists will refuse their compliance with the supposi¬ 
tion that they were formed as our crater-bearing 
volcanoes were formed: and we have pointed out 

* American Journal of Science , Second Series, vol. ii. 


202 THE GREAT RING-FORMATIONS [chap. ix. 

the logical impossibility of assigning any limit of 
size beyond which the eruptive action could not be 
said to hold good, so long as the central cone is 
present. But when we come to ring-mountains 
having no cones, and of such enormous size that 
we are compelled to hesitate in ascribing them to 
ejective action, we are obliged to face the possibility 
of some other causation. And, failing an explana¬ 
tion of our own that satisfied us, we have alluded to 
the few hypotheses proffered by others, and of these 
Professor Dana's appears the most rational, since it 
is based upon a parallel found on the earth. In 
citing it, however, we do not necessarily indorse it. 



Plate XX.—Mercator and Campanus. 


10 6 o 

L. 

vilES 


10 20 30 tO SO 60 70 

1-1-1_I_l_i_l 

Scale 


[To face page 202 . 





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CHAPTER X 


PEAKS AND MOUNTAIN RANGES 

The lunar features next in order of conspieuity are 
the mountain ranges, peaks, and hill-chains, a class 
of eminences more in common with terrestrial for¬ 
mations than the craters and circular structures 
that have engaged our notice in the preceding 
chapters. 

In turning our attention to these features, we are 
at the outset struck with the paucity on the lunar 
surface of extensive mountain systems as compared 
with its richness in respect of crateral formations; 
and a field of speculation is opened by the recog¬ 
nition of the remarkable contrast which the moon 
thus presents to the earth, where mountain ranges 
are the rule, and craters like the lunar ones are 
decidedly exceptional. Another conspicuous but 
inexplicable fact is that the most important ranges 
upon the moon occur in the northern half of the 

203 


204 PEAKS AND MOUNTAIN RANGES [chap. 

visible hemisphere, where the craters. are fewest 
and the comparatively featureless districts termed 
“seas” are found. The finest range is that named 
after our Apennines and which is included in our 
illustrative Plate, No. XXII. It extends for about 
450 miles, and has been estimated to contain up¬ 
wards of 3000 peaks, one of which — Mount 
Huyghens—attains the altitude of 18,000 feet. The 
Caucasus is another lunar range which appears like 
a diverted northward extension of the Apennines, 
and, although a far less imposing group than the 
last named, contains many lofty peaks, one of which 
approaches the altitude assigned to Mount Huyghens 
while several others range between 11,000 and 
14,000 feet high. Another considerable range is 
the Alps, situated between the Caucasus and the 
crater Plato, and reproduced on Plate XII. It 
contains some 700 peaked mountains, and is re¬ 
markable for the immense valley, 80 miles long and 
about 5 broad, that cuts it with seemingly arti¬ 
ficial straightness; and that, were it not for the 
flatness of its bottom, might set one speculating 
upon the probability of some extraneous body hav¬ 
ing rushed by the moon at an enormous velocity, 
gouging the surface tangentially at this point and 
cutting a channel through the impeding mass of 


X.] 


ISOLATED PEAKS 


205 


mountains. There are other mountain ranges of 
less magnitude than the foregoing; but those we 
have specified will suffice to illustrate our sugges¬ 
tions concerning this class of features. 

We remark, too, that there is a prevailing 
tendency of the ranges just mentioned to present 
their loftiest constituents in abrupt terminal lines, 
facing nearly the same direction, the reverse of that 
towards which they are carried by the moon’s rota¬ 
tion ; and as they recede from the high terminal 
line, the mountains gradually fall off in height, so 
that in bulk the ranges present the “crag and tail” 
contour which individual hills upon the earth so 
frequently exhibit. 

Isolated peaks are found in small numbers upon 
the moon; there are a few striking examples of 
them nevertheless, and these are chiefly situated in 
the mountainous region just alluded to. Several 
are seen to the east (right hand) of the Alpine range 
depicted on Plate XII. The best known of these 
is Pico, which rises abruptly from a generally smooth 
plain to a height of 7000 feet. It may be recognised 
as the lower of the two long shadowing spots located 
almost centrally above the crater Plato in the illus¬ 
tration just mentioned. Above it, at an actual 
distance of 40 miles, there is another peak (un- 


206 PEAKS AND MOUNTAIN RANGES [chap. 

named), about 4000 feet high; and away to the 
west, beyond the small crater joined by a hill-ridge 
to Plato, is a third pyramidal mountain nearly as 
high as Pico. 

It seems natural to regard the great mountain 
chains as agglomerations of those peaks of which we 
have isolated examples in Pico and its compeers, 
and thus to consider that the formation of a 
mountain chain has been a multiplication of the 
process that formed the single pyramid-shaped 
eminences. At first thought it might appear that 
the great mountain ranges were produced by bodily 
upthrustings of the crust of the moon by some sub¬ 
surface convulsions. But such an explanation could 
hardly hold in relation to the isolated peaks, for it 
is difficult, if not impossible, to conceive that these 
abrupt mountains, almost resembling a sugarloaf in 
steepness, could have been protruded en masse 
through a smooth region of the crust. On the con¬ 
trary, it is quite consistent with probability to 
suppose that they were built up by a slow process 
somewhat analogous to that to which we have 
ascribed the piling of the central cones of the greater 
craters. We believe they may be regarded as true 
mountains of exudation, produced by the compara¬ 
tively gentle oozing of lava from a small orifice and 



[To face page 206 - 


Plate XXI.—An ideal sketch of “Pico,” an isolated lunar mountain 8000 feet high, as it 
would probably appear if seen by a spectator located on the Moon, 





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[To face page 207 . 







X.] 


LAVA MOUNTAINS 


207 


its solidification around it; the vent however re¬ 
maining open and the summit or discharging orifice 
continually rising with the growth of the mountain, 
as indicated in Fig. 36. This process is well ex¬ 
emplified in the case of a water fountain playing 
during a severe frost; the water as it falls around 
the lips of the orifice freezes into a hillock of ice, 
through the centre of which, however, a vent for 
the fluid is preserved. As the water trickles over 
the mound it is piled higher and higher by accumu¬ 
lating layers of ice, till at length a massive cone is 
formed whose height will be determined by the 
force or “head” of the water. Substitute lava 
for water, and we have at once a formative process 
which may very fairly be considered as that which 
has given rise to the isolated mountains of the 
moon. 

There are upon the earth mountainous forms re¬ 
sembling the isolated peaks of the moon, and which 
have been explained by a similar theory to the 
above. We reproduce a figure of one observed by 
Dana at Hawaii (Fig. 37), and a sketch of another 
observed on the summit of the volcano of Bourbon 
(Fig. 38); we also reproduce (Fig. 39) an ideal 
section of the latter, given by Mr Scrope, and show¬ 
ing the successive layers of lava which would be 


208 PEAKS AND MOUNTAIN RANGES [chap. 

disposed by just such an action as that manifested 
in the case of the freezing fountain; and we quote 
that author’s words in reference to this explana¬ 
tion of the formation of Etna and other volcanic 
mountains. “On examining,” says Mr Scrope, # 
“ the structure of the mountain (Etna) we find its 
entire mass, so far as it is exposed to view by 
denudation or other causes (and one enormous 
cavity, the Yal de Bove penetrates deeply into its 
very heart), to be composed of beds of lava-rock 
alternating more or less irregularly with layers of 
scorise, lapillo and ashes, almost precisely identical 
in mineral character, as well as in general disposi¬ 
tion, with those erupted by the volcano at known 
dates within the historical period. Hence we are 
fully justified in believing the whole mountain to 
have been built up in the course of ages in a similar 
manner by repeated intermittent eruptions. And 
the argument applies by the rules of analogy to all 
other volcanic mountains, though the history of 
their recent eruptions may not be so well recorded, 
provided that their structure corresponds with, and 
can be fairly explained by, this mode of produc¬ 
tion. It is also further applicable, under the same 
reservation, to all mountains composed entirely, or 
* Volcanoes, page 155. 



[To face page, 208. 


















































































f 








[To face page 209 . 


Small Volcanic Mountain at the end of a street at Teneriffe. 
































































































































































































X.] 


A “BLOWING CONE 


209 


for the most part, of volcanic rocks, even though 
they may not have been in eruption within our 
time.” 

To these illustrations furnished from Scrope’s 
work we add another, copied from a photograph 
by Professor Piazzi Smyth, of a “ blowing cone ” at 
the base of Teneriffe (Fig. 40), which is but one of 
many that are to be found on that mountain, and 
which has been formed by a process similar to that 
we have been considering, but acting upon a com¬ 
paratively small scale. Professor Smyth describes 
this cone as about 70 feet high and of parabolic 
figure, composed of hard lava and with an upper 
aperture still yawning, “ whence the burning breath 
of fires beneath once issued in fury and with 
destruction.” 

Reverting now to the moon, we remark that, if 
the foregoing explanation of the isolated lunar peaks 
be tenable, it should hold equally for the groups of 
them which we see in the lunar Apennines, Alps, 
Caucasus, and other ranges of like character. There 
occur in some places intermediate groups which 
link the one to the other. Just above the crater 
Archimedes, on Plate XXII., for instance, we see 
several single peaks and small clumps of them 
leading by successive multiple-peak examples to 


210 


PEAKS AND MOUNTAIN RANGES [chap. 


what may be called chains of mountains like many 
that are included in the contiguous Apennine system. 
And, in view of this connection between the single 
peaks and the mountain ranges formed of aggrega¬ 
tions of such peaks, it seems to us reasonable to 
conclude that the latter were formed by the com¬ 
paratively slow escape of lava through multitudinous 
openings in a weak part of the moon’s crust, rather 
than to suppose that the crust itself has been bodily 
upheaved and retained in its disturbed position. 
The high peaks that many mountains in such a 
chain exhibit accord better with the former than 
the latter explanation; for it is difficult to 
imagine how such lofty eminences could be 
erected by an upheaval, and we must remember 
that the moon has none of the denuding elements 
which are at work upon the earth, weather- 
wearing its mountain forms into sharpness and 
steepness.* 

And we have ground for believing the mountain¬ 
forming process on the moon to have been a com- 

* In reference to such prominences on the lunar surface as 
cast steeple-like shadows, it is well to remark that we must not 
in all cases infer, from the acute spire-like form of the shadow, 
that the object which casts the shadow is of a similar sharp or 
spire-like form, which the first impression would naturally lead 
us to suppose. A comparatively blunt or rounded eminence 



Plate XXII.—The Lunar Apennines, Archimedes, etc., etc. 


JO O ro 20 30 40 SO 60 70 BO 90 TOO no 720 120 790 ISO 
‘-‘-J-1-*-1-1-1- L L 1-1-1_I |_l_1_J 

MILES 

Scale. 


[To face page 210. 
















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X.] 


BUILDING THE MOUNTAIN 


211 


paratively gentle one, in the fact that the mountain 
systems appear in regions otherwise little disturbed, 
and where craters, which have all the appearances 
of violent origin, are few and far between. Evi¬ 
dently the mountain and crater-forming processes, 
although both due to extrusive action, were in some 
measure different, and it is reasonable to suppose 
that the difference was in degree of intensity; so 
that while a violent ejection of volcanic material 
would give rise to a crater, a more gradual discharge 
would pile up a mountain. In this view craters are 

will project a long and pointed shadow when the rays of light 
fall on the object at a low angle, and especially so when the 
shadow is projected on a convex surface. We illustrate this 
with a copy of an actual photograph of the shadow cast by half 
a pea, Fig. 41. 



Fig. 41. 




212 


PEAKS AND MOUNTAIN RANGES [chap. 


evidences of eruptive , and mountains of compara¬ 
tively gentle exudative action. 

We can hardly speculate with any degree of 
safety upon the cause of this varying intensity of 
volcanic discharge. We may ascribe it to variation 
of depth of the initial disturbing force, or to sudden¬ 
ness of its action; or it may be that different 
degrees of fluidity of the lava have had modifying 
effects; or on the other hand different qualities of 
the crust-material; or yet again differences of period 
—the quieter extrusions having occurred at a time 
when the volcanic forces were dying down. There 
is an alliance between lunar craters and mountains 
that goes far to show that there has been no radical 
difference in their origins. For instance, as we 
have previously pointed out, craters in some cases 
run in linear groups, as if in those cases they had 
been formed along a line of disruption or of least 
resistance of the crust; and the mountain chains 
have a corresponding linear arrangement. Then 
we see craters and mountain chains disposed in what 
seem obviously the same arcs of disturbance. Thus 
Copernicus (No. 147), Erastothenes (No. 168), and 
the Apennines appear to belong to one continuous 
line of eruption; and it requires no great stretch of 
imagination to suppose that the Caucasus, Eudoxus 


X.] 


RIDGES AND CRACKS 


213 


(No. 208), and Aristotle (No. 209) form a continua¬ 
tion of the same line. Then around the Mare 
Serenetatis we see mountainous ridges and craters 
alternating one with the other as though the exud¬ 
ing action there, normally sufficient to produce the 
ridges, had at some points become forcible enough 
to produce a crater. Again, upon the very mountain 
ranges themselves, as, for instance, among the Apen¬ 
nines, we find small craters occurring. We see, 
too, that the great craters are in many cases sur¬ 
rounded by radiating systems of ridges which almost 
assume mountainous proportions, and which are 
doubtless exuded matter from “starred” cracks, 
the centres of which are occupied by the craters. 
The same kind of ridges here and there occur apart 
from craters (see for instance Plate XIV., below 
Aristarchus and Herodotus) and sometimes they 
occur in the neighbourhood of extensive cracks, to 
which they also seem allied. We must indeed 
regard a linear crack as the origin either of a ridge 
(if the exudation is slight) or of a mountain chain 
(if the exudation is more copious) or a string of 
craters (if the extrusion rises to eruptive violence). 
But the subject of cracks is important enough to 
be treated in a separate chapter. 

We alluded in Chap. III. to the phenomena of 


214 PEAKS AND MOUNTAIN RANGES [chap. 

wrinkling or puckering as productive of certain 
mountainous formations; and we pointed out the 
striking similarity in character of configuration be¬ 
tween a shrivelled skin and a terrestrial mountain 
region. We do not perceive upon the moon such a 
decided coincidence of appearances extending over 
any considerable portion of her surface; but there 
are numerous limited areas where we behold 
mountainous ridges which partake strongly of the 
wrinkle character; and in some cases it is difficult 
to decide whether the puckering agency or the 
exudative agency just discussed has produced the 
ridges. The district bordering upon Aristarchus 
and Herodotus, above referred to, is of this doubtful 
character; and a similar district is that contiguous 
to Triesnecker (Plate IX.). There are, however, 
abundant examples of less prominent lines of eleva¬ 
tion, which may, with more probability, be ascribed 
to a veritable wrinkling or puckering action; they 
are found over nearly the whole lunar surface, some 
of them standing out in considerable relief, and 
some merely showing gentle lines of elevation, or 
giving the surface an undulating appearance. A 
close examination of our picture map (Plate V.) will 
reveal very numerous examples, especially in the 
south-east (right-hand-upper) quadrant. Some of 


X.] 


LINES OF TUMESCENCE 


215 


these lines of tumescence are so slightly prominent 
that we may suppose them to have been caused by 
the action indicated by Fig. 6 (facing p. 48), while 
others, from their greater boldness, appear to 
indicate a formative action analogous to that 
represented by Fig. 9. 


CHAPTER XI 


CRACKS AND RADIATING STREAKS 

We have hitherto confined our attention to those 
reactions of the moon’s molten interior upon its 
exterior which have been accompanied by consider¬ 
able extrusions of sub-surface material in its molten 
or semi-solid condition. We now pass to the con¬ 
sideration of some phenomena resulting in part from 
that reaction and in part from other effects of cool¬ 
ing, which have been accompanied by comparatively 
little ejection or upflow of molten matter, and in 
some cases by none at all. Of such the most con¬ 
spicuous examples are those bright streaks that are 
seen, under certain conditions of illumination, to 
radiate in various directions from single craters, and 
some of the individual radial branches of which 
extend from four to seven hundred miles in a great 
arc on the moon’s surface. 

There are several prominent examples of these 

216 


chap, xi.] BRIGHT STREAK SYSTEMS 


217 


bright streak systems upon the visible hemisphere 
of the moon ; the focal craters of the most conspic¬ 
uous are Tycho, Copernicus, Kepler, Aristarchus, 
Menelaus, and Proclus. Generally these focal 
craters have ramparts and interiors distinguished 
by the same peculiar bright or highly reflective 
material which shows itself with such remarkable 
brilliance, especially at full moon : under other con¬ 
ditions of illumination they are not so strikingly 
visible. At or nearly full moon the streaks are 
seen to traverse over plains, mountains, craters, 
and all asperities; holding their way totally dis¬ 
regardful of every object that happens to lay in 
their course. 

The most remarkable bright streak system is 
that diverging from the great crater Tycho. The 
streaks that can be easily individualised in this 
group number more than one hundred, while the 
courses of some of them may be traced through up¬ 
wards of six hundred miles from their centre of 
divergence. Those around Copernicus, although 
less remarkable in regard to their extent than those 
diverging from Tycho, are nevertheless in many 
respects well deserving of careful examination : they 
are so numerous as utterly to defy attempts to 
count them, while their intricate reticulation renders 


218 CRACKS AND RADIATING STREAKS [chap. 


any endeavour to delineate their arrangement equally 
hopeless. 

The fact that these bright streaks are invariably 
found diverging from a crater, impressively indicates 
a close relationship or community of origin between 
the two phenomena : they are obviously the result 
of one and the same causative action. It is no less 
clear that the actuating cause or prime agency must 
have been very deep-seated and of enormous dis¬ 
ruptive power to have operated over such vast areas 
as those through which many of the streaks extend. 
With a view to illustrate experimentally what we 
conceive to have been the nature of this actuating 
cause, we have taken a glass globe, and, having filled 
it with water and hermetically sealed it, have 
plunged it into a warm bath: the enclosed water, 
expanding at a greater rate than the glass, exerts a 
disruptive force on the interior surface of the latter, 
the consequence being that at the point of least re¬ 
sistance, the globe is rent by a vast number of 
cracks diverging in every direction from the focus of 
disruption. The result is such a strikingly similar 
counterpart of the diverging bright streak systems 
which we see proceeding from Tycho and the other 
lunar craters before referred to, that it is impossible 
to resist the conclusion that the disruptive action 








Plate XXIII.—Glass Globe, cracked by internal pressure, illustrating the 
cause of the bright streaks radiating from Tycho. 


[To face page 218. 









- 









. 





















■ 







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Plate XXIY. — Full Moon, exhibiting the bright streaks radiating 

from Tycho. 


[To lace page 218, 



















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XI.] 


THEIR CAUSATIVE ORIGIN 


219 


which originated them operated in the same manner 
as in the case of our experimental illustration; the 
disruptive force in the case of the moon being that 
to which we have frequently referred as due to the 
expansion which precedes the solidification of 
molten substances of volcanic character. 

On Plate XXIII. we present a photograph from 
one of many glass globes which we have cracked in 
the manner described : a careful comparison between 
the arrangement of divergent cracks displayed by 
this photograph with the streaks seen spreading 
from Tycho upon the contiguous lunar photograph 
will, we trust, justify us in what we have stated as 
to the similarity of the causes which have produced 
such identical results. 

The accompanying figures will further illustrate 
our views upon the causative origin of the bright 
streaks. The primary action rent the solid crust of 
the moon and produced a system of radiating fissures 
(Fig. 42): these immediately afforded egress for the 
molten matter beneath to make its appearance on 
the surface simultaneously along the entire course 
of every crack, and irrespective of all surface in¬ 
equalities or irregularities whatever (Fig. 43). We 
conceive that the upflowing matter spread in both 
directions sideways, and in this manner produced 


220 CRACKS AND RADIATING STREAKS [chap. 

streaks of very much greater width than the cracks 
or fissures up through which it made its way to the 
surface. 

In further elucidation of this part of our subject, 
we may refer to a familiar but as we conceive cogent 
illustration of an analogous action in the behaviour 
of water beneath the ice of a frozen pond, which, 
on being fractured by some concentrated pressure, 
or by a blow, is well known to “ star ” into radiating 
or diverging cracks, up through which the water 
immediately issues, making its appearance on the 
surface of the ice simultaneously along the entire 
course of every crack, and on reaching the surface, 
spreading on both sides to a width much exceeding 
that of the crack itself. 

If this familiar illustration be duly considered, 
we doubt not it will be found to throw considerable 
light on the nature of those actions which have re¬ 
sulted in the bright streaks on the moon’s surface. 
Some have attempted to explain the cause of these 
bright streaks by assigning them to streams of lava, 
issuing from the crater at the centre of their diver¬ 
gence and flowing over the surface, but we consider 
such an explanation totally untenable, as any idea 
of lava, be it ever so fluid at its first issue from its 
source, flowing in streams of nearly equal width, 



Fig. 42. 

Illustrative of the Radiating Cracks which precede the formation of the 
Bright Streaks. 


[To face page 220. 

































Fig. 43. 

Illustrative of the Radiating Bright Streaks. 


[To face page 220. 






















































* 










XL] 


NO SHADOWS VISIBLE 


221 


through courses several hundred miles long, up 
hills, over mountains, and across plains, appears to 
us beyond all rational probability. 

It may be objected to our explanation of the 
formation of these bright streaks, that so far as our 
means of observation avail us, we fail to detect any 
shadows from them or from such marginal edges as 
might be expected to result from a side-way spread¬ 
ing outflow of lava from the cracks which afforded 
it exit in the manner described. Were the edges of 
these streaks terminated by cliff-like or craggy mar¬ 
gins of such height as 30 or 40 feet, we might just 
be able at low angles of illumination and under the 
most favourable circumstances of vision, to detect 
some slight appearance of shadows; but so far as 
we are aware, no such shadows have been observed. 
We are led to suppose that the impossibility of 
detecting them is due not to their absence but to 
the height of the margins being so moderate as not 
to cast any cognisable shadow, inasmuch as an 
abrupt craggy margin of 10 or 15 feet high would, 
under even the most favourable circumstances, fail 
to render such visible to us. Reference to our ideal 
section of one of these bright streaks (Fig. 45) will 
sljow how thin their edges may be in relation to 
their spreading width. 


222 CRACKS AND RADIATING STREAKS [chap. 

The absence of cognisable shadows from the 
bright streaks has led some observers to conclude 
that they have no elevation above the surface over 
which they traverse, and it has therefore been sug¬ 
gested that their existence is due to possible vapours 
which may have issued through the cracks, and 
condensed in some sublimated or pulverulent form 
along their courses, the condensed vapours in 
question forming a surface of high reflective pro¬ 
perties. That metallic or mineral substances of 
some kinds do deposit on condensation very white 
powders, or sublimates, we are quite ready to admit, 
and such explanation of the high luminosity of the 
bright streaks, and of the craters situated at the foci 
or centres of their divergence is by no means im¬ 
probable, so far as concerns their mere brightness. 
But as we invariably find a crater occupying the 
centre of divergence, and such craters are possessed 
of all the characteristic features and details which 
establish their true volcanic nature as the results of 
energetic extrusions of lava and scoria, we cannot 
resist the conclusion that the material of the crater, 
and that of the bright streaks diverging from it, are 
not only of a common origin, but are so far identical 
that the only difference in the structure of the one 
as compared with the other is due to the more 


XL] 


NARROW CRACKS OR CHASMS 


223 


copious egress of the extruded or erupted matter in 
the case of the crater, while the restricted outflow 
or ejection of the matter up through the cracks 
would cause its dispersion to be so comparatively 
gentle as to flood the sides of the cracks and spread 
in a thin sheet more or less sideways simultaneously 
along their courses. There are indeed evidences in 
the wider of the bright streaks of their being the 
result of the outflow of lava through systems of 
cracks running parallel to each other, the confluence 
of the lava issuing from which would naturally yield 
the appearance of one streak of great width. Some 
of those diverging from Tycho are of this class; 
many other examples might be cited, among which 
we may name the wide streaks proceeding from the 
crater Menelaus and also those from Proclus. Some 
of these occupy widths upwards of 25 miles—amply 
sufficient to admit of many concurrent cracks with 
confluent lava outflows. 

We are disposed to consider as related to the 
forementioned radiating streaks, the numerous, we 
may say the multitudinous, long and narrow chasms 
that have been sometimes called “canals ” or “rills,” 
but which are so obviously cracks or chasms, that it 
is desirable that this name should be applied to 
them rather than one which may mislead by imply- 


224 CRACKS AND RADIATING STREAKS [chap. 


ing an aqueous theory of formation. These cracks, 
singly and in groups, are found in great numbers in 
many parts of the moon’s surface. As a few of the 
more conspicuous examples which our plates exhibit, 
we may refer to the remarkable group west of 
Triesnecker (Plate IX.), the principal members of 
which converge to or cross at a small crater, and 
thus point to a continuity of causation therewith 
analogous to the evident relation between the bright 
streaks and their focal craters. Less remarkable, 
but no less interesting, are those individual examples 
that appear in the region north of (below) the Apen¬ 
nines (Plate XXII.), and some of which by their 
parallelism of direction with the mountain-chain 
appear to point to a causative relation also. There 
is one long specimen, and several shorter in the 
immediate neighbourhood of Mercator and Cam- 
panus (Plate XX.); and another curious system of 
them, presenting suggestive contortions, occurs in 
connection with the mountains Aristarchus and 
Herodotus (Plate XIV.). Others, again, appear to 
be identified with the radial excrescences about 
Copernicus (Plate VI.). Capuanus, Agrippa, and 
Gassendi, among other craters, have more or less 
notable cracks in the ; r vicinities. 

Some of these chasms are conspicuous enough to 


XI.] 


CHASMS 


225 


be seen with moderate telescopic means, and from 
this maximum degree of visibility there are all grades 
downwards to those that require the highest optical 
powers and the best circumstances for their detec¬ 
tion. The earlier selenographers detected but a few 
of them. Schroeter noted only 11; Lohrman re¬ 
corded 75 more; Beer and Maedler added 55 to the 
list, while Schmidt of Athens raised the known 
number to 425, of which he has published a descrip¬ 
tive catalogue. We take it that this increase of 
successive discoveries has been due to the pro¬ 
gressive perfection of telescopes, or, perhaps, to 
increased education, so to speak, of the eye, since 
Schmidt's telescope is a much smaller instrument 
than that used by Beer and Maedler, and is regarded 
by its owner as an inferior one for its size. We 
doubt not that there are hundreds more of these 
cracks which more perfect instruments and still 
sharper eyes will bring to knowledge in the future. 

While these chasms have all lengths from 150 
miles (which is about the extent of those near 
Triesnecker) down to a few miles, they appear to 
have a less variable breadth, since we do not find 
many that at their maximum openings exceed two 
miles across; about a mile or less is their usual 
width throughout the greater part of their length, 


226 CRACKS AND RADIATING STREAKS [chap. 


and generally they taper off to invisibility at their 
extremities, where they do not encounter and ter¬ 
minate at a crater or other asperity, which is, how¬ 
ever, sometimes the case. Of their depth we can 
form no precise estimate, though from the sharpness 
of their edges we may conclude that their sides 
approach perpendicularity, and, therefore, that their 
depth is very great; we have elsewhere suggested 
ten miles as a possible profundity. In a few cases, 
and under very favourable circumstances, we have 
observed their generally black interiors to be inter¬ 
rupted here and there with bright spots suggestive 
of fragments from the sides of the cracks having 
fallen into the opening. 

In seeking an explanation of these cracks, two 
possible causes suggest themselves. One is the 
expansion of sub-surface matter, already suggested 
as explanatory of the bright streaks; the other, a 
contraction of the crust by cooling. We doubt not 
that both causes have been at work, one perhaps 
enhancing the other. Where, as in the cases we 
have pointed out, there are cracks which are so 
connected with craters as to imply relationship, we 
may conclude that an upheaving or expansive force 
in the sublunar molten matter has given rise to the 
cracks, and that the central craters have been formed 


XI.] 


EFFECTS OF SOLIDIFICATION 


227 


simultaneously, by the release, with ejective violence, 
of the matter from its confining crust. The nature 
of the expansive force being assumed that of solidi¬ 
fying matter, the wide extent of some chasms indi¬ 
cates a deep location of that force. And depth in 
this matter implies lateness (in the scale of seleno- 
logical time) of operation, since the central portions 
of the globe would be the last to cool. Now, we 
have evidence of comparative lateness afforded by 
the fact that in many cases the cracks have passed 
through craters and other asperities which thus 
obviously existed before the cracking commenced; 
and thus, so far, the hypothesis of the expansion¬ 
cracking is supported by absolute fact. 

It may be objected that such an upheaving force 
as we are invoking, being transitory, would allow 
the distended surface to collapse again when it 
ceased to operate, and so close the cracks or chasms 
it produced. But we consider it not improbable 
that in some cases, as a consequence of the expan¬ 
sion of sub-surface matter, an upflow thereof may 
have partially filled the crack, and by solidifying 
have held it open; and it is rational to suppose that 
there have been various degrees of filling and even 
of overflow—that in some cases the rising matter 
has not nearly reached the edge of the crack, as 


228 CRACKS AND RADIATING STREAKS [chap. 


in Fig. 44, while in others it has risen almost to 
the surface, and in some instances has actually 
overrun it and produced some sort of elevation 
along the line of the crack, like that represented 
sectionally in Fig. 45. It is probable that some of 
the slightly tumescent lines on the moon’s surface 
have been thus produced. 

We have suggested shrinkage as a possible ex¬ 
planation of some cracks. It could hardly have 
been the direct cause of those compound ones which 
are distinguished by focal craters, though it may 
have been a co-operative cause, since the contracting 
tendency of any area of the crust, by so to speak 
weakening it, may have virtually increased the 
strength of an upheaving force and thus have aided 
and localised its action. We see, however, no 
reason why the inevitable ultimate contraction 
which must have attended the cooling of the moon’s 
crust, even when all internal reactions upon it had 
ceased, should not have created a class of cracks 
without accompanying craters, while it would doubt¬ 
less have a tendency to increase the length and 
width of those already existing from any other 
cause. Some of the more minute clefts, which pre¬ 
sumably exist in greater numbers than we yet know 
of, may doubtless be ascribed to this effect of cool- 



J.N 

Fig. 44. 



Fig. 45. 


* 


\To face page 228. 

















XI.] 


A DIFFERENCE IN SUMMITS 


229 


ing contraction. In this view we should have to 
regard such cracks as the latest of all lunar features. 
Whether the agency that produced them is still at 
work—whether the cracks are on the increase—is a 
question impossible of solution: for reasons to be 
presently adduced, we incline to believe that all 
cosmical heat passed from the moon, and therefore 
that it arrived at its present, and apparently final, 
condition ages upon ages ago. 

Besides the ridges spoken of on p. 226, and re¬ 
garded as cracks up through which matter has been 
extruded, there are numerous ridges of greater or 
less extent, which we conceive are of the nature of 
wrinkles, and have been produced by tangential 
compression due to the collapse of the moon’s crust 
upon the shrunken interior, as explained and illus¬ 
trated in Chap. III. The distinguishing feature of 
the two classes of phenomena we consider to be the 
presence of a serrated summit in those of the ex¬ 
truded class, while those produced by “ wrinkling ” 
action have their summits comparatively free from 
serration or marked irregularity. 


CHAPTER XII 


COLOUR AND BRIGHTNESS OF LUNAR DETAILS : 

CHRONOLOGY OF FORMATIONS, AND FINALITY OF 

EXISTING FEATURES 

Speaking generally, the details of the lunar surface 
seem to us to be devoid of colour. To the naked 
eye of ordinary sensitiveness the moon appears to 
possess a silvery whiteness : more critical judges of 
colour would describe it as presenting a yellowish 
tinge. Sir John Herschel, during his sojourn at 
the Cape of Good Hope, had frequent opportunities 
of comparing the moon’s lustre with that of the 
weathered sandstone surface of Table Mountain, 
when the moon was setting behind it, and both were 
illuminated under the same direction of sunlight; 
and he remarked that the moon was at such times 
“ scarcely distinguishable from the rock in apparent 
contact with it.” Although his observations had 
reference chiefly to brightness, it can hardly be 


chap, xii.] COLOUR OF THE MOON 231 

doubted that similarity of colour is also implied; 
for any difference in the tint of the two objects 
would have precluded the use of the words “ scarcely 
distinguishable ”; a difference of colour interfering 
with a comparison of lustre in such an observation, 
though it must be remembered that he observed 
through a dense stratum of atmosphere. Viewed 
in the telescope, the same general yellowish-white 
colour prevails over all the moon, with a few excep¬ 
tions offered by the so-called seas. The Mare 
Crisium , Mare Serenetatis, and Mare Humorum 
have somewhat of a greenish tint; the Palus Somnii 
and the circular area of Lichtenberg incline to ruddi¬ 
ness. These tints are, however, extremely faint, 
and it has been suggested by Arago that they may 
be mere effects of contrast rather than actual colour¬ 
ation of the surface material. This, however, can 
hardly be the case, since all the “ seas ” are not alike 
affected; those that are slightly coloured are, as we 
have said, some green and some red, and contrast 
could scarcely produce such variations. The sup¬ 
position of vegetation covering these great flats and 
giving them a local colour is in our view still more 
untenable, in the face of the arguments that we 
shall presently adduce against the possibility of 
vegetable life existing upon the moon. 


232 


LUNAR DETAILS 


[chap. 


It appears to us more rational to consider the 
tints due to actual colour of the material (presum¬ 
ably lava or some once fluid mineral substance) that 
has covered these areas; and it may well be con¬ 
ceived that the variety of tint is due to different 
characters of material, or even various conditions 
of the same material coming from different depths 
below the lunar surface; and we may reasonably 
suppose that the same variously-coloured substances 
occur in the rougher regions of the lunar surface, 
but that they exist there in patches too small to be 
recognised by us, or are “ put out ” by the brightness 
to which polyhedral reflexion gives rise. 

Seeing that volcanic action has had so large a 
share in giving to the moon’s surface its structural 
character, analogy of the most legitimate order justi¬ 
fies us in concluding not only that the materials of 
that surface are of kindred nature to those of the 
unquestionably volcanic portions of the earth, but 
also that the tints and colours that characterise 
terrestrial volcanic and Plutonian products have 
their counterparts on the moon. Those who have 
seen the interior and surroundings of a terrestrial 
volcano after a recent eruption, and before atmos¬ 
pheric agents have exercised their dimming influ¬ 
ences, must have been struck with the colours of 


XII.] 


THE LOCAL COLOURS 


233 


the erupted materials themselves and the varied 
brilliant tints conferred on these materials by the 
sublimated vapours of metals and mineral sub¬ 
stances which have been deposited upon them. If, 
then, analogy is any guide in enabling us to infer 
the appearance of the invisible from that which we 
know to be of kindred nature and which we have 
seen, we may justly conclude that were the moon 
brought sufficiently near to us to exhibit the minute 
characteristics of its surface, we should behold the 
same bright and varied colours in and around its 
craters that we behold in and about those of the 
earth ; and in all probability the coloured materials 
of lunar volcanoes would be more fresh and vivid 
than those of the earth by reason of the absence of 
those atmospheric elements which tend so rapidly 
to impair the brightness of coloured surfaces ex¬ 
posed to their influence. 

Situated as we are, however, as regards distance 
from the moon, we have no chance of perceiving 
these local colours in their smaller masses; but it 
is by no means improbable, as we have suggested, 
that the faint tints exhibited by the great plains are 
due to broad expanses of coloured volcanic material. 

But if we fail to perceive diversity of colour upon 
the lunar surface, we are in a very different position 


234 


LUNAR DETAILS 


[CHAP. 


in regard to diversity of brightness or variable light- 
reflective power of different districts and details. 
This will be tolerably obvious to those casual ob¬ 
servers, who have remarked nothing more of the 
moon’s physiography than the resemblance to a 
somewhat lugubrious human countenance which the 
full moon exhibits, and which is due to the acci¬ 
dental disposition of certain large and small areas of 
surface material which have less of the light-reflect¬ 
ing property than other portions; for since all parts 
seen by a terrestrial observer may be said to be 
equally shone upon by the sun, it is clear that ap¬ 
parently bright and shaded parts must be produced 
by differences in the nature of the surface as regards 
power of reflecting the light received. 

When we turn to the telescope and survey the 
full disc of the moon with even a very moderate 
amount of optical aid, the meagre impression as to 
variety of degree of brightness which the unassisted 
eye conveys is vastly extended and enhanced, for 
the surface is seen to be diversified by shades of 
brilliancy and dulness from almost glittering white 
to sombre grey: and this variety of shading is ren¬ 
dered much more striking by shielding the eye with 
a dusky glass from the excessive glare, which drowns 
the details in a flood of light. Under these circum- 


XII.] SPLASHES OF BRIGHTNESS 235 

stances the varieties of light and shade become 
almost bewildering, and defy the power of brush or 
pencil to reproduce them. 

We may, however, realise an imperfect idea of 
this characteristic of the lunar surface by reference 
to the self-drawn portrait of the full moon upon 
Plate IY. This is, in fact, a photograph taken from 
the full moon itself, and enlarged sufficiently to 
render conspicuous the spots and large and small 
regions that are strikingly bright in comparsion with 
what may in this place be described as the “ ground ” 
of the disc. As an example of a wide and irregu¬ 
larly extensive district of highly reflective material, 
the region of which Tycho is the central object, is 
very remarkable. We may refer also to the bright 
“ splashes ” of which Copernicus and Kepler are the 
centres. So brilliant are these spots that they can 
easily be detected by the unassisted eye about the 
time of full moon. Still brighter but less conspicu¬ 
ous by its size is the crater Aristarchus, which shines 
with specular brightness, and almost induces the 
belief that its interior is composed of some vitreous¬ 
surfaced matter: the highly-reflective nature of this 
object has often caused it to become conspicuous 
when in the dark hemisphere of the moon, unil¬ 
luminated by the sun, and lighted only by the light 


236 


LUNAR DETAILS 


[chap. 


reflected from the earth. At these times it appears 
so bright that it has been taken for a volcano in 
actual eruption, and no small amount of popular 
misconception at one time arose therefrom concern¬ 
ing the conditions of the moon as respects existing 
volcanic activity—a misconception that still clings to 
the mind of many. 

The parts of the surface distinguished by defici¬ 
ency of reflecting power are conspicuous enough. 
We may cite, however, as an example of a detailed 
portion especially remarkable for its dingy aspect, 
the interior of the crater Plato, which is one of the 
darkest spots (the darkest well-defined one) upon the 
hemisphere of the moon visible to us. For facili¬ 
tating reference to shades of luminosity, Schroeter 
and Lolirman assorted the variously reflective parts 
into 10 grades, commencing with the darkest. 
Grades 1 to 8 comprised the various deep greys; 
4 and 5 the light greys; 6 and 7 white; and 8 to 
10 brilliant white. The spots Grimaldi and Riccioli 
came under class 1 of this notation; Plato between 
1 and 2. The “ seas ” generally ranged from 2 to 3 ; 
the brightest mountainous portions mostly between 
degrees 4 and 6; the crater walls and the bright 
streaks came between these and the bright peaks, 
which fell under the 9th grade. The maximum 


XII.] CHRONOLOGICAL ORDER OF FORMATION 237 

brightness, the 10th grade, is instanced only in the 
case of Aristarchus and a point in Werner, though 
Proclus nearly approaches it, as do many bright 
spots, chiefly the sites of minute craters, which make 
their appearance at the time of full moon. 

In photographic pictures produced by the moon 
of itself there is always an apparent exaggeration in 
the relation of light to dark portions of the disc. 
The dusky parts look, upon the photograph, much 
darker than to the eye directed to the moon itself, 
whether assisted or not by optical appliances. It 
may be that the real cause of this discrepancy is 
that the eye fails to discover the actual difference 
upon the moon itself, being insensible to the higher 
degrees of brightness or not estimating them at 
their proper brilliance with respect to parts less 
bright. On the other hand, it is probable that the 
enhanced contrast in the photograph is due to some 
peculiar condition of the darker surface matter 
affecting its power of reflecting the actinic constitu¬ 
ent of the rays that fall upon it. 

The study of the varying brightness or reflective 
power of different regions and spots of the lunar 
disc leads us to the consideration of the relative 
antiquity of the surface features; for it is hardly 
possible to regard these variations attentively with- 


238 


LUNAR DETAILS 


[chap. 


out being impressed with the conviction that they 
have relation to some chronological order of forma¬ 
tion. We cannot, in the first place, resist the con¬ 
viction that the brightest features were the latest 
formed; this strikes us as evident on prima fade 
grounds; but it becomes more clearly so when we 
remark that the bright formations, as a rule, overlie 
the duller features. The elevated parts of the crust 
are brighter than the “ seas ” and other areas; and 
it is pretty clear that the former are newer than the 
latter, upon which they appear to be super-imposed, 
or through which they seem to have extruded. * The 
vast dusky plains are in every instance more or less 
sprinkled with spots and minute craters, and these 
last were obviously formed after the area that con¬ 
tains them. One is almost disposed to place the 
order of formations in the order of relative bright¬ 
ness, and so consider the dingiest parts the oldest 
and the brightest spots and craters the newest 
features, though, in the absence of an atmosphere 
competent to impair the reflective power of the 
surface materials, we are unable to justify this 

* We meet a difficulty in reconciling this idea with the partial 
craters of which we have a conspicuous example in Fracastorius, 
No. 78, of our Map, which seem to be partially sunk below the 
contiguous surface. This looks as though the crater-rim belonged 
to an older epoch than the plain from which it rises. 


XII.] 


RELATIVE AGES OF CRATERS 


239 


classification by suggesting a cause for such a 
deterioration by time as the hypothesis pre¬ 
supposes. 

As we have entered upon the question of relative 
age of the lunar features, we may remark that there 
are evidences of various epochs of formation of par¬ 
ticular classes of details, irrespective of their con¬ 
dition in respect of brightness, or, as we may say, 
freshness of material. As a rule, the large craters 
are older than the small ones. This is proved by 
the fact that a large object of this class is never 
seen to interfere with or overlap a small one. 
Those of nearly equal size are, however, seen to 
overlap one another as though several eruptions of 
equal intensity had occurred from the same source 
at different points. This is strikingly instanced in 
the group of craters situated in the position 35—141 
on our map, the order of formation of each of which 
is clearly apparent. The region about Tycho offers 
an inexhaustible field for study of these phenomena 
of overlapping or interpolating craters, and it will 
be found, with very few exceptions, that the smaller 
crater is the impinging or parasitical one, and must 
therefore have been formed after the larger, upon 
which it intrudes or impinges. There are frequent 
cases in which a large crater has had its rampart 


240 


LUNAR DETAILS 


[chap. 


interrupted by a lesser one, and this again has been 
broken into by one still smaller; and instances may 
be found where a fourth crater smaller than all has 
intruded itself upon the previous intruder. The 
general tendency of these examples is to show that 
the craters diminished in size as the moon’s volcanic 
energy subsided : that the largest were produced in 
the throes of its early violence, and that the smallest 
are the results of expiring efforts possibly impeded 
through the deep-seatedness of the ejective source. 

Another general fact of this chronological order 
is that the mountain chains are never seen to in¬ 
trude upon formations of the crater order. We do 
not anywhere find that a mountain chain runs 
absolutely into or through a crater; but, on the 
other hand, we do find that craters have formed on 
mountain chains. This leads unmistakably to the 
inference that the craters were not formed before 
their allied mountain chains ; and we might assume 
therefore that the mountains generally are the older 
formations, but that there is nothing to prove that 
the two classes of features, where they intermingle, 
as in the Apennines and Caucasus, were not erupted 
contemporaneously. 

Upon the assumption that the latest ejected or 
extruded matter is that which is brightest, we should 


XII.] THE CRACKS LATE PHENOMENA 


241 


place the bright streaks among the more recent 
features. Be this as it may, it is tolerably certain 
that the cracks, whose apparently close relation to 
the radiating streaks we have endeavoured to point 
out, are relatively of a very late formative period. 
We are indeed disposed to consider them as the 
most recent features of all: the evidence in support 
of this consideration being the fact that they are 
sometimes found intersecting small craters that, 
from the way in which they are cut through by the 
cracks, must have been in situ before the cracking 
agency came into operation. It is in accordance 
with our hypothesis of the moon’s transition from a 
fluid to a solid body to consider that a cracking of 
the surface would be the latest of all the pheno¬ 
mena produced by contraction in final cooling. 

The foregoing remarks naturally lead us to the 
question whether changes are still going on upon 
the surface of our satellite: whether there is still 
left in it a spark of its volcanic activity, or whether 
that activity has become totally extinct. We shall 
consider this question from the observational and 
theoretical point of view. First as regards observa¬ 
tions. This much may be affirmed indisputably— 
that no object or detail visible to the earliest seleno- 
graphers (whose period may be dated 200 years 

Q 


242 


LUNAR DETAILS 


[chap. 


back) has altered from the date of their maps to the 
present. When we pass from the bolder features to 
the more minute details we find ourselves at a loss 
for materials for forming an inference ; the only map 
pretending to accuracy even of the larger among 
small objects being that of Beer and Maedler, 
which, truly admirable as it is, is not very safely to 
be relied upon for settling any question of alleged 
change, on account of the conventional system 
adopted for exhibiting the forms of objects, every 
object being mapped rather than drawn, and shown 
as it never is or can be presented to view on the 
moon itself. This difficulty would present itself if 
a question of change were ever raised upon the 
evidence of Beer and Maedler’s map : it may indeed 
have prevented such a question being raised, for 
certainly no one has hitherto been bold enough to 
assert that any portion or detail of the map fails to 
represent the actual state of the moon at the present 
time. 

In default of published maps, we are thrown for 
evidence on this question upon observations and re¬ 
collections of individual observers whose familiarity 
with the lunar details extends over lengthy periods. 
Speaking for ourselves, and upon the strength of 
close scrutinies continued with assiduity through 


XII.] 


LINNE 


243 


the past thirty years, we may say that we have 
never had the suspicion suggested to our eye of 
any actual change whatever having taken place in 
any feature or minute detail of the lunar surface; 
and our scrutinies have throughout been made with 
ample optical means, mostly with a 20-inch reflector. 
This experience has made us not unnaturally in 
some slight decree sceptical concerning the changes 
alleged to have been detected by others. Those 
asserted by Schroeter and Gruithuisen were long 
ago rejected by Beer and Maedler, who explained 
them, where the accuracy of the observer was not 
questioned, by variations of illumination, a cause of 
illusory change which is not always sufficiently 
taken into account. A notable instance of this 
deception occurred a few years ago in the case of 
the minute bright crater Linne , which was for a 
considerable period declared, upon the strength of 
observations of very promiscuous character, to be 
varying in form and dimensions almost daily, but 
the alleged constant changes of which have since 
been tacitly regarded as due to varying circum¬ 
stances of illumination induced by combinations of 
libratory effects with the ordinary changes depend¬ 
ing upon the direction of the sun’s rays as due to 
the age of the moon. This explanation does not, 


244 


LUNAR DETAILS 


[chap. 


however, dispose of the question whether the crater 
under notice suffered any actual change before the 
hue and cry was raised concerning it. Attention 
was first directed to it by Schmidt, of Athens, 
whose powers of observation are known to be re¬ 
markable, and whose labours upon the moon are 
of such extent and minuteness as to claim for his 
assertions the most respectful consideration. * He 
affirmed in 1866 that the crater at that date pre¬ 
sented an appearance decidedly different from that 
which it had had since 1841 : that whereas it had 
been from the earlier epoch always easily seen as a 
very deep crater, in October 1866, and thence¬ 
forward it presented only a white spot, with at most 
but a very shallow aperture, very difficult to be 
detected. Schmidt is one of the very few observers 
whose long familiarity with the moon entitles him 
to speak with confidence upon such a question as 
that before us upon the sole strength of his own 
experience; and this case is but an isolated one, at 

* We are informed by a friend, who has lately visited Athens, 
that Schmidt’s detail drawings of the moon, comprising the 
work of forty years, form a small library in themselves. The 
map embodying them is so large (6 ft. 6 in. in diameter) and so 
full of detail that there is small hope of its complete publication, 
unless there should be such a wide extension of interest in the 
minute study of our satellite as to justify the cost of reproduc¬ 
ing it. 




[To face ptvgt 244. 
























XII.] NO VESTIGE OF PRESENT CHANGE 245 


least it is the only one he has brought forward. 
He is, however, still firmly convinced that it is an 
instance of actual change, and not an illusion re¬ 
sulting from some peculiar condition of illumination 
of the object. It should be added also on this side 
of the discussion that an English observer, the 
Rev. T. W. Webb, while apparently indisposed to 
concede the supposition of any notable changes in 
the lunar features, has yet found from his own 
observations that, after all due allowance for differ¬ 
ences of light and shade upon objects at different 
times, there is still a “ residuum of minute variations 
not thus disposed of” which seem to indicate that 
eruptive action In the moon has not yet entirely 
died out, though its manifestation at present is very 
limited in extent. It appears to us that, if evidence 
of continuing volcanic action is to be sought on the 
moon, the place to look for it is around the circum¬ 
ference of the disc, where eruptions from any mar¬ 
ginal orifice would manifest itself in the form of a 
protruding haziness, somewhat as illustrated to an 
exaggerated extent in the annexed cut (Fig. 46). 

The theoretical view of the question, which we 
have now to consider, has led us, however, to the 
strong belief that no vestige of its former volcanic 
activity lingers in the moon—that it assumed its 


246 


LUNAR DETAILS 


[chap. 


final condition an inconceivable number of ages ago, 
and that the high interest which would attach to 
the close scrutiny of our satellite if it were still the 
theatre of volcanic reaction cannot be hoped for. 
If it be just and allowable to assume that the earth 
and the moon were condensed into planetary form 
at nearly the same epoch (and the only rational 
scheme of cosmogony justifies the assumption) then 
we may institute a comparison between the con¬ 
dition of the two bodies as respects their volcanic 
age, using the one as a basis for inference concern¬ 
ing the state of the other. We have reason to 
believe that the earth’s crust has nearly assumed 
its final state so far as volcanic reactions of its 
interior upon its exterior are concerned: we may 
affirm that within the historical period no igneous 
convulsions of any considerable magnitude have 
occurred; and we may consider that the volcanoes 
now active over the surface of the globe represent 
the last expiring efforts of its eruptive force. Now 
in the earth we perceive several conditions where¬ 
from we may infer that it parted with its cosmical 
heat (and therefore with its prime source of volcanic 
agency) at a rate which will appear relatively very 
slow when we come to compare the like conditions in 
the moon. We may, we think, take for granted that 


XII.] 


RETENTION OF HEAT 


247 


the surface of a planetary body generally determines 
its heat dispersing power, while its volume deter¬ 
mines its heat retaining power. Given two spheri¬ 
cal bodies of similar material but of unequal magni¬ 
tude and originally possessing the same degree of 
heat, the smaller body will cool more rapidly than 
the larger, by reason of the greater proportion 
which the surface of the smaller sphere bears to its 
volume than that of the larger sphere to its volume 
—this proportion depending upon the geometrical 
ratio which the surfaces of spheres bear to their 
volumes, the contents of spheres being as the cubes 
and the surfaces as the squares of their diameters. 
The volume of the earth is 49 times as great as 
that of the moon, but its surface is only 18 times 
as great; there is consequently in the earth a power 
of retaining its cosmical heat nearly four times as 
great as in the case of the moon; in other words, 
the moon and earth being supposed at one time to 
have had an equally high temperature, the moon 
would cool down to a given low temperature in 
about one-fourth the time that the earth would 
require to cool to the same temperature. But the 
earth’s cosmical heat has without doubt been con¬ 
siderably conserved by its vaporous atmosphere, 
and still more by the ocean in its antecedent vapor- 


248 


LUNAR DETAILS 


[CHAP. 


ous form. Yet notwithstanding all this, the earth's 
surface has nearly assumed its final condition so far 
as volcanic agencies are concerned: it has so far 
cooled as to be subject to no considerable distor¬ 
tions or disruptions of its surface. What then must 
be the state of the moon, which, from its small 
volume and large proportionate area, parted with 
its heat at the above comparatively rapid rate ? 
The matter of the moon is, too, less dense than the 
earth, and from this cause doubtless disposed to 
more rapid cooling; and it has no atmosphere or 
vaporous envelope to retard its radiating heat. We 
are driven thus to the conclusion that the moon's 
loss of cosmical heat must have been so rapid as to 
have allowed its surface to assume its final con¬ 
formation ages on ages ago, and hence that it is 
unreasonable and hopeless to look for evidence of 
change of any volcanic character still going on. 

We conceive it possible, however, that minute 
changes of a non-volcanic character may be pro¬ 
ceeding in the moon, arising from the violent alter¬ 
nations of temperature to which the surface is ex¬ 
posed during a lunar day and night. The sun, as 
we know, pours down its heat unintermittingly for 
a period of fully 300 hours upon the lunar surface, 
and the experimental investigations of I<ord Rosse, 



XII.] 


LUNAR HEAT AND COLD 


249 


essentially confirmed by those of the French obser¬ 
ver, Marie Davy, show that under this powerful 
insolation the surface becomes heated to a degree 
which is estimated at about 500° of Fahrenheit’s 
scale, the fusing point of tin or bismuth. This 
heat, however, is entirely radiated away during the 
equally long lunar night, and, as Sir John Herschel 
surmised, the surface probably cools down again to 
a temperature as low as that of interstellar space : 
this has been assumed as representing the absolute 
zero of temperature which has been calculated from 
experiments to be 250° below the zero of Fahren¬ 
heit’s scale. Now such a severe range of heat and 
cold can hardly be without effect upon some of the 
component materials of the lunar surface/' If 
there be any such materials as the vitreous lavas 
that are found about our volcanoes, such as obsidian 
for instance, they are doubtless cracked and shivered 
by these extreme transitions of temperature; and 
this comparatively rapid succession of changes con¬ 
tinued through long ages would, we may suppose, 
result in a disintegration of some parts of the 
surface and at length somewhat modify the seleno- 

* It is conceivable that the alleged changes in the crater Linne 
may have been caused by a filling of the crater by some such 
crumbling action as we are here contemplating. 


250 


LUNAR DETAILS 


[chap. 


graphic contour. It is, however, possible that the 
surface matter is mainly composed of more crystal¬ 
line and porous lavas, and these might withstand 
the fierce extremes like the “ fire-brick ” of mundane 
manufacture, to which in molecular structure they 
may be considered comparable. Lavas as a rule 
are (upon the earth) of this unvitreous nature, and 
if they are of like constitution on the moon, there 
will be little reason to suspect changes from the 
cause we are considering. Where, however, the 
material, whatever its nature, is piled in more or 
less detached masses, there will doubtless be a 
grating and fracturing at the points of contact of 
one mass with another, produced by alternate ex¬ 
pansions and contractions of the entire masses, 
which in the long run of ages must bring about dis¬ 
locations or dislodgments of matter that might 
considerably affect the surface features from a close 
point of view, but which can hardly be of sufficient 
magnitude to be detected by a terrestrial observer 
whose best aids to vision give him no perception of 
minute configurations. And it must always be 
borne in mind that changes can only be proved by 
reference to previous observations and delineations 
of unquestionable accuracy. 

Speaking by our own lights, from our own ex- 


XII.] 


THE UNCHANGING MOON 


251 


perience and reasoning, we are disposed to conclude 
that in all visible aspects the lunar surface is un¬ 
changeable, that in fact it arrived at its terminal 
condition aeons of ages ago, and that in the survey 
of its wonderful features, even in the smallest 
details, we are presented with the sight of objects 
of such transcendent antiquity as to render the 
oldest geological features of the earth modern by 
comparison. 


CHAPTER XIII 


THE MOON AS A WORLD : DAY AND NIGHT UPON ITS 
SURFACE 

A wide interest, if not a deep one, attaches to the 
general question as to the existence of living beings, 
or at least the possibility of organic existence, on 
planetary bodies other than our own. The question 
has been examined in all ages, by the lights of the 
science peculiar to each. With every important 
accession to our astronomical knowledge it has been 
re-raised : every considerable discovery has given 
rise to some new step or phase in the discussion, 
and in this way there has grown up a somewhat ex¬ 
tensive literature exclusively relating to mundane 
plurality. It will readily be understood that the 
moon, from its proximity to the earth, has from the 
first received a large, perhaps the largest, share of 
attention from wanderers in this field of speculation : 
and we might add greatly to the bulk of this volume 

252 


CHAP. XIII.] 


LIFE AND THE MOON 


253 


by merely reviewing some of the more curious and, 
in their way, instructive conjectures specially relat¬ 
ing to the moon as a world—to imaginary journeys 
towards her, and to the beings conjectured to dwell 
upon and within her. This, however, we feel there 
is no occasion to do, for it is our purpose merely to 
point out the two or three almost conclusive argu¬ 
ments against the possibility of any life, animal or 
vegetable, having existence on our satellite. 

We well know what are the requisite conditions 
of life on the earth; and we can go no further for 
grounds of inference; for if we were to start by 
assuming forms of life capable of existence under 
conditions widely and essentially different from 
those pertaining to our planet, there would be no 
need for discussing our subject further: we could 
revel in conjectures, without a thought as to their 
extravagance. The only legitimate phase of the 
question we can entertain is this :—can there be on 
the moon any kind of living things analogous to any 
kind of living things upon the earth? And this 
question, we think, admits only of a negative 
answer. The lowest forms of vitality cannot exist 
without air, moisture, and a moderate range of 
temperature. It may be true, as recent experi¬ 
ments seem to show, that organic germs will retain 


254 


THE MOON AS A WORLD 


[chap. 


their vitality without either of the first, and with 
exposure to intense cold and to a considerable 
degree of heat; and it is conceivable that the mere 
germs of life may be present on the moon.* But 
this is not the case with living organisms themselves. 
We have, in Chapter V., specially devoted to the 
subject, cited the evidence from which we know that 
there can be at the most, no more air on the moon 
than is left in the receiver of an air-pump after the 
ordinary process of exhaustion. And with regard 
to moisture, it could not exist in any but the vapor¬ 
ous state, and we know that no appreciable amount 
of vapour can be discovered by any observation (and 
some of them are crucial enough) that we are cap¬ 
able of making. We may suppose it just within the 
verge of possibility that some low forms of vegeta- 

* Is it not conceivable that the protogerms of life pervade 
the whole universe, and have been located upon every planetary 
body therein ? Sir William Thomson’s suggestion that life came 
to the earth upon a seed-bearing meteor was weak, in so far that 
it shifted the locus of life-generation from one planetary body 
to another. Is it not more philosophical, more consistent with 
our conception of Creative omnipotence and impartiality, to sup¬ 
pose that the protogerms of life have been sown broadcast over 
all space, and that they have fallen here upon a planet under 
conditions favourable to their development, and have sprung 
into vitality when the fit circumstances have arrived, and there 
upon a planet that is, and that may be for ever, unfitted for their 
vivification. 


XIII.] 


HUMAN LIFE IMPOSSIBLE 


255 


tion might exist upon the moon with a paucity of 
air and moisture such as would be beyond even our 
most severe powers of detection : but granting even 
this, we are met by the temperature difficulty; for 
it is inconceivable that any plant-life could survive 
exposure first to a degree of cold vastly surpassing 
that of our arctic regions, and then in a short time 
(14 days) to a degree of heat capable of melting the 
more fusible metals—the total range being equal, as 
we have elsewhere shown, to perhaps 600 or 700 
degrees of our thermometric scale. 

The higher forms of vegetation could not reason¬ 
ably be expected to exist under conditions which 
the lower forms could not survive. And as regards 
the possibility of the existence of animal life in any 
form or condition on the lunar surface, the reasons 
we have adduced in reference to the non-existence 
of vegetable life bear still more strongly against the 
possibility of the existence of the former. We know 
of no animal that could live in what may be con¬ 
sidered a vacuum and under such thermal conditions 
as we have indicated. 

As to man, aeronautic experiences teaches us 
that human life is endangered when the atmosphere 
is still sufficiently dense to support 12 inches of 
mercury in the barometer tube; what then would 


256 


THE MOON AS A WORLD 


[chap. 


be his condition in a medium only sufficiently dense 
to sustain one-tenth of an inch of the barometric 
column ? We have evidence from the most delicate 
tests that no atmosphere or vapour approaching 
even this degree of attenuation exists around the 
moon's surface. 

Taking all these adverse conditions into con¬ 
sideration, we are in every respect justified in con¬ 
cluding that there is no possibility of animal or 
vegetable life existing on the moon, and that our 
satellite must therefore be regarded as a barren 
world. 

After this disquisition upon lunar uninhabit¬ 
ability it may appear somewhat inconsistent for us 
to attempt a description of the scenery of the moon 
and some other effects that would be visible to a 
spectator, and of which he would be otherwise 
sensible, during a day and a night upon her surface. 
But we can offer the sufficient apology that an 
imaginary sojourn of one complete lunar day and 
night upon the moon affords an opportunity of 
marshalling before our readers some phenomena 
that are proper to be noticed in a work of this char¬ 
acter, and that have necessarily been passed over 
in the series of chapters on consecutive and special 


XIII.] 


A FLIGHT OF FANCY 


257 


points that have gone before. It may be urged that, 
in depicting the moon from such a standpoint as 
that now to be taken, we are describing scenes that 
never have been such in the literal sense of the word, 
since no eye has ever beheld them. Still we have 
this justification—that we are invoking the concep¬ 
tion of things that actually exist; and that we are 
not, like some imaginary voyagers to the moon, in¬ 
dulging in mere flights of fancy. Although it is 
impossible for a habitant of this earth fully to realise 
existence upon the moon, it is yet possible, indeed 
almost inevitable, for a thoughtful telescopist — 
watching the moon night after night, observing the 
sun rise upon a lunar scene, and noting the course 
of effects that follow till it sets—it is almost inevit¬ 
able, we say, for such an observer to identify him¬ 
self so far with the object of his scrutiny, as some¬ 
times to become in thought a lunar being. Seated 
in silence and in solitude at a powerful telescope, 
abstracted from terrestrial influences, and gazing 
upon the revealed details of some strikingly char¬ 
acteristic region of the moon, it requires but a small 
effort of the imagination to suppose one’s self actually 
upon the lunar globe, viewing some distant land¬ 
scape thereupon; and under these circumstances 
there is an irresistible tendency in the mind to pass 


258 


THE MOON AS A WORLD 


[chap. 


beyond the actually visible , and to fill in with what 
it knows must exist those accessory features and 
phenomena that are only hidden from us by distance 
and by our peculiar point of view. Where the 
material eye is baffled, the clairvoyance of reason 
and analogy comes to its aid. 

Let us then endeavour to realise the strange con¬ 
sequences which the position and conditions of the 
moon produce upon the aspect of a lunar landscape 
in the course of a lunar day and night. 

The moon’s day is a long one. From the time 
that the sun rises upon a scene * till it sets, a period 
of 804 hours elapses, and of course double this 
interval passes between one sunrise and the next. 
The consequences of this slow march of the sun 
begin to show themselves from the instant that he 
rises above the lunar horizon. Dawn, as we have 
it on earth, can have no counterpart upon the moon. 
No atmosphere is there to reflect the solar beams 
while the luminary is yet out of actual sight, and 
only the glimmer of the zodiacal light heralds the 
approach of day. From the black horizon the sun 
suddenly darts his bright untempered beams upon 

* Our remarks have general reference to a region of the moon 
near her equator; near the poles some of the conditions we shall 
describe would be somewhat modified. 


XIII.] SUNRISE AND DAWN 259 

the mountain tops, crowning them with dazzling 
brilliance while their flanks and valleys are yet in 
utter darkness. There is no blending of the night 
into day. And yet there is a growth of illumina¬ 
tion that in its early stages may be called a twilight, 
and which is caused by the slow rise of the sun. 
Upon the earth, in central latitudes, the average 
time occupied by the sun in rising, from the first 
glint of his upper edge till the whole disc is in sight, 
is but two minutes and a quarter. Upon the moon, 
however, this time is extended to a few minutes 
short of an hour, and therefore, during the first few 
minutes a dim light will be shed by the small visible 
chord of the solar disc, and this will give a pro¬ 
portionately modified degree of illumination upon 
the prominent portion of the landscape, and impart 
to it something of the weird aspect which so strikes 
an observer of a total solar eclipse on earth when 
the scene is lit by the thin crescent of the re¬ 
appearing sun. This impaired illumination con¬ 
stitutes the only dawn that a lunar spectator could 
behold. And it must be of short duration ; for when, 
in the course of half an hour, the solar disc has risen 
half into view the lighting would no doubt appear 
nearly as bright to the eye as when the entire disc 
of the sun is above the horizon. In this lunar sun- 


260 


THE MOON AS A WORLD 


[CHAP. 


rise, however, there is none of that gilding and 
glowing which makes the phenomenon on earth so 
gorgeous. Those crimson sky-tints with which we 
are familiar are due to the absorption of certain of the 
polychromous rays of light by our atmosphere. The 
blue arid violet components of the solar beams are 
intercepted by our envelope of vapour, and only the 
red portions are free to pass; while on the moon, 
as there is no atmosphere, this selective absorption 
does not occur. If it did, an observer gazing from 
the earth upon the regions of the moon upon which 
the sun is just rising would see the surface tinted 
with rosy light. This, however, is not the case : the 
faintest lunar features just catching the sun are seen 
simply under white light diluted to a low degree of 
brightness. Only upon rare occasions is the lunar 
scenery suffused with coloured illumination, and 
these are when, as we shall presently have to 
describe, the solar rays reach the moon after tra¬ 
versing the earth’s atmosphere during an eclipse of 
the sun. 

This atmosphere of ours is the most influential 
element in beautifying our terrestrial scenery, and 
the absence of such an appendage from the moon is 
the great modifying cause that affects lunar scenery 
as compared with that of the earth. We are accus- 


XIII.] BLAZING SUN AND BLACK SKY 261 

tomed to tlie sun with its dazzling brightness- 
overpowering though it be—subdued and softened 
by our vaporous screen. Upon the moon there is 
no such modification. The sun’s intrinsic brilliancy 
is undiminished, its apparent distance is shortened, 
and it gleams out in fierce splendour only to be 
realised, and then imperfectly, by the conception of 
a gigantic electric light a few feet from the eye. 
And the brightness is rendered the more striking by 
the blackness of the surrounding sky. Since there 
is no atmosphere there can be no sky-light, for there 
is nothing above the lunar world to diffuse the solar 
beams; not a trace of that moisture which even in 
our tropical skies scatters some of the sun’s light 
and gives a certain degree of opacity or blueness, 
deep though it be, to the heavens by day. Upon 
the moon, with no light-diffusing vapour, the sky 
must be as dark or even darker than that with 
which we are familiar upon the finest of moonless 
nights. And this blackness prevails in the full 
blaze of the lunar noon-day sun. If the eye (upon 
the moon) could bear to gaze upon the solar orb 
(which Avould be less possible than upon earth) or 
could it be screened from the direct beams, as 
doubtless it could by intervening objects, it would 
perceive the nebulous and other appendages which 


262 


THE MOON AS A WORLD 


[chap. 


we know as the corona, the zodiacal light, and the 
red solar protuberances: or if these appendages 
could not be viewed with the sun above the horizon 
they would certainly be seen in glorious perfection 
when the luminary was about to rise or immediately 
after it had set. 

And, notwithstanding the sun’s presence, the 
planets and stars would be seen to shine more 
brilliantly than we see them on the clearest of 
nights ; the constellations would have the same con¬ 
figurations, though they would be differently situated 
with respect to the celestial pole about which they 
would appear to turn, for the axis of rotation of the 
moon is directed towards a point in the constellation 
Draco. The stars would never twinkle or change 
colour as they appear to us to do, for scintillation or 
twinkling is a phenomenon of atmospheric origin, 
and they would retain their full brightness, down 
even to the horizon, since there would be no haze to 
diminish their light. The planets, and the brighter 
stars at least, would be seen even when they were 
situated very near to the sun. The planet Mercury, 
so seldom detected by terrestrial gazers, would be 
almost constantly in view during the lunar day, 
manifesting his close attendance on the central 
luminary by making only short excursions of about 


XIII.] 


THE PLANETS 


263 


two (lunar) days’ length, first on one side and then 
on the other. Venus would be nearly as continu¬ 
ously visible, though her wanderings would be more 
extensive on either side. The zodiacal light also, 
which in our English latitude and climate is but 
rarely seen and in more favourable climes appears 
only when the sun itself is hidden beneath the 
horizon, would upon the moon be seen as a constant 
accompaniment to the luminary throughout his daily 
course across the lunar sky. The other planets 
would appear generally as they do to us on earth, 
but, never being lost in daylight, their courses 
among the stars could be traced with scarcely any 
interruption. 

One planet, however, that adorns the sky of 
the lunar hemisphere which is turned towards us 
deserves special mention from the conspicuous and 
highly interesting appearance it must present. We 
allude to the earth. To nearly one-half of the moon 
(that which we never see) this imposing object can 
never be visible; but to the half that faces us the 
terrestrial planet must appear almost fixed in the 
sky. A lunar spectator in (what is to us) the centre 
of the disc, or about the region north of the lunar 
mountains Ptolemy and Hipparchus, would have 
the earth in his zenith. From regions upon the 


264 THE MOON AS A WORLD [chap. 

moon a little out of what is to us the centre, a 
spectator would see the earth a little declining from 
the zenith, and this declination would increase as 
the regions corresponding to the (to us) apparent 
edge of the moon were approached, till at the actual 
edge it would be seen only upon the horizon. From 
the phenomena of libration (explained in Chapter 
VI.) the earth would appear from nearly all parts of 
the lunar hemisphere to which it is visible at all to 
describe a small circle in the sky. To an observer, 
however, upon the (to us) marginal regions of the 
lunar globe, it would appear only during a portion 
of the lunar day—being visible in fact only in that 
part of its small circular path which happened to lie 
above the observer’s horizon: in some regions only 
a portion of the terrestrial disc would make its brief 
appearance. From the lunar hemisphere beyond 
this marginal line the earth can never be seen at all. 

The lunar spectator whose situation enabled him 
to view the earth would see it as a moon; and a 
glorious moon indeed it must be. Its diameter 
would be four times as great as that of the moon 
itself as seen by us, and the area of its full disc 18 
times as great. It would be seen to pass through 
its phases, just as does our satellite, once in a lunar 
day or a terrestrial month, and during that cycle of 


XIII.] 


THE EARTH-MOON 


265 


phases, since 29 of our days would be occupied by 
it, the axial rotation would bring all the features of 
its surface configuration into view so many times in 
succession. But the greatest beauty of this noble 
moon would be seen during the lunar night, in con¬ 
sidering which we shall again allude to it; for when 
it is full-moon to the earth it is new-earth to the 
moon. At lunar midnight this globe of ours is fully 
illuminated; as morning nears, the earth-moon 
wanes, its disc slowly passing through the gibbous 
phases until at sunrise it would be just half-illumi¬ 
nated. During the long forenoon it assumes a 
crescent which narrows and narrows till at midday 
the sun is in line with the earth and the latter is in¬ 
visible, save perhaps by a thin line of light marking 
its upper or lower edge, accordingly as the sun is 
apparently above or below it. In the lunar after¬ 
noon an illuminated crescent appears upon the 
opposite side of the terrestrial globe, and this widens 
and widens till it becomes a half disc by lunar sun¬ 
set and a full disc by lunar midnight. 

The sun in his daily course passes at various 
distances, sometimes above and sometimes below, 
the nearly stationary earth. Obviously it will at 
times pass actually behind it, and then the lunar 
spectator would behold the sublime spectacle of a 


266 THE MOON AS A WORLD [chap. 

total solar eclipse, and that under circumstances 
which render the phenomenon far more imposing 
than its counterpart can appear from the earth ; 
for whereas, when we see the moon eclipse the sun, 
the nearly similar (apparent) diameters of the two 
bodies render the duration of totality extremely 
short—at most 7 minutes—a lunar spectator, the 
earth appearing to him four times the diameter of 
the sun, and he and the earth being relatively 
stationary, would enjoy a view of the totality ex¬ 
tending over several hours. During the passage of 
the solar disc behind that of the earth, a beautiful 
succession of luminous phenomena would be ob¬ 
served to follow from the refractions and dispersions 
which the sunbeams would suffer in passing tan¬ 
gentially through those parts of our atmospheric 
envelope which lie in their course; those, for 
instance, on the margin of the earth, as seen from 
the moon. As the sun passed behind the earth, 
the latter would be encircled upon the in-going side 
with a beautiful line of golden light, deepening in 
places to glowing crimson, due to the absorption, 
already spoken of, of all but the red and orange 
rays of the sun’s light by the vapours of our atmos¬ 
phere. As the eclipse proceeded and totality came 
on, this ruddy glow would extend itself nearly, if 


XIII.] THE EARTH FROM THE MOON 267 

not all, around the black earth, and so bright would 
it be, that the whole lunar landscape covered by 
the earth’s shadow would be illuminated with faint 
crimson light,* save, perhaps, in some parts of the 
far distance, upon which the earth had not yet cast 
its shadow, or off which the shadow had passed. 
Although the crimson light would preponderate, it 
would not appear bright and red alike all around 
the earth’s periphery. The circle of light would be, 
in fact, the ring of twilight round our globe, and it 
would only appear red in those places where the 
atmosphere chanced to be in that condition favour¬ 
able for producing what on earth we know as red 
sunset and sunrise. We know that the sun, even 
in clear sky, does not always set and rise with the 
beautiful red glow, which may be determined by 
merely local causes, and will therefore vary in 
different parts of the earth. Now a lunar spectator 
watching the sun eclipsed by the earth, would see, 
during totality and at a coup cVoeil , every point 

* We see this reddening during an eclipse of the moon (when 
the event we are describing—an eclipse of the sun visible from 
the moon—really takes place). The blood-red colour has often 
struck observers very forcibly, and it has indeed been suggested 
that the appearance may be the innocent and oft-repeated fulfil¬ 
ment of the prophetic allusion to the moon being “turned into 
blood/' 


268 


THE MOON AS A WORLD 


[chap. 


around our world upon which the sun is setting on 
one side and rising upon the other. To every part 
of the earth around what is then the margin, as 
seen from the moon, the sun is upon the horizon, 
shining through a great thickness of atmosphere, 
reddening it, and being reddened by it wherever 
the vaporous conditions conduce to that coloration. 
And at all parts where these conditions obtain, the 
lunar eclipse-observer would see the ring of light 
around the black earth-globe brilliantly crimsoned; 
at other parts it would have other shades of red 
and yellow, and the whole effect would be to make 
the grand earth-ball, hanging in the lunar sky, like a 
dark sphere in a circle of glittering gold and rubies. 

During the early stages of the eclipse, this chap¬ 
let of brilliant-coloured lights would be brightest 
upon the side of the disappearing sun; at the time 
of central eclipse the radiance (supposing the sun 
to pass centrally behind the earth) would be equally 
distributed, and during the later stages it would 
preponderate upon the side of the reappearing sun. 
We have endeavoured to give a pictorial realisation 
of this phenomenon and of the effect of the eclipse 
upon the lunar landscape, but such a picture cannot 
but fall very, very far short of the reality. (See 
the Frontispiece.) 


XIII.] 


LUNAR SCENERY 


269 


And now for a time let us turn attention from 
the lunar sky to the scenery of the lunar landscape. 
Let us, in imagination, take our stand high upon 
the eastern side of the rampart of one of the great 
craters. Height, it must be remarked, is more 
essential on the moon to command extent of view 
than upon the earth, for on account of the compara¬ 
tive smallness of the lunar sphere the dip of the 
horizon is very rapid. Such height, however, would 
be attained without great exercise of muscular 
power, since equal amounts of climbing energy 
would, from the smallness of lunar gravity, take a 
man six times as high on the moon as on the earth. 
Let us choose, for instance, the hill-side of Coper¬ 
nicus. The day begins by a sudden transition. The 
faint looming of objects under the united illumina¬ 
tion of the half-full earth, and the zodiacal light is 
the lunar precursor of daybreak. Suddenly the 
highest mountain peaks receive the direct rays of 
a portion of the sun’s disc as it emerges from 
below the horizon. The brilliant lighting of these 
summits serves but to increase, by contrast, the 
prevailing darkness, for they seem to float like 
islands of light in a sea of gloom. At a rate of 
motion twenty-eight times slower than we are 
accustomed to, the light tardily creeps down the 


270 


THE MOON AS A WORLD 


[chap. 


mountain-sides, and in the course of about twelve 
hours the whole of the circular rampart of the great 
crater below us, and towards the east, shines out 
in brilliant light, unsoftened by a trace of mountain- 
mist. But on the opposite side, looking into the 
crater, nothing but blackness is to be seen. As 
hour succeeds hour, the sunbeams reach peak after 
peak of the circular rampart in slow succession, till 
at length the circle is complete and the vast crater- 
rim, 50 miles in diameter, glistens like a silver- 
margined abyss of darkness. By-and-by, in the 
centre, appears a group of bright peaks or bosses. 
These are the now illuminated summits of the 
central cones, and the development of the great 
mountain cluster they form henceforth becomes an 
imposing feature of the scene. From our high 
standpoint, and looking backwards to the sunny 
side of our cosmorama, we glance over a vast region 
of the wildest volcanic desolation. Craters from 
five miles diameter downwards crowd together in 
countless numbers, so that the surface, as far as the 
eye can reach, looks veritably frothed over with 
them. Nearer the base of the rampart on which 
we stand, extensive "mountain chains run to north 
and to south, casting long shadows towards us; 
and away to southward run several great chasms a 


XIII.] DESOLATION AND LIFELESSNESS 


271 


mile wide and of appalling blackness and depth. 
Nearer still, almost beneath us, crag rises on crag 
and precipice upon precipice, mingled with craters 
and yawning pits, towering pinnacles of rock and 
piles of scorise and volcanic debris. But we behold 
no sign of existing or vestige of past organic life. 
No heaths or mosses soften the sharp edges and 
hard surfaces : no tints of cryptogamous or lichenous 
vegetation give a complexion of life to the hard fire- 
worn countenance of the scene. The whole land¬ 
scape, as far as the eye can reach, is a realisation 
of a fearful dream of desolation and lifelessness— 
not a dream of death, for that implies evidence of 
pre-existing life, but a vision of a world upon which 
the light of life has never dawned. 

Looking again, after some hours’ interval, into 
the great crateral amphitheatre, we see that the rays 
of the morning sun have crept down the distant 
side of the rampart, opposite to that on which we 
stand, and lighted up its vast landslipped terraces 
into a series of seeming hill-circles with all the rude 
and rugged features of a terrestrial mountain view, 
and none of the beauties save those of desolate 
grandeur. The plateau of the crater is half in 
shadow 10,000 feet below, with its grand group of 
cones, now fully in sight, rising from its centre. 


272 THE MOON AS A WORLD [chap. 

Although these last are twenty miles away and the 
base of the opposite rampart fully double that 
distance, we have no means of judging their remote¬ 
ness, for in the absence of an atmosphere there can 
be no aerial perspective, and distant objects appear 
as brilliant and distinct as those which are close to 
the observer. Not the brightness only, but the 
various colours also of the distant objects are pre¬ 
served in their full intensity; for colour we may 
fairly assume there must be. Mineral chlorates and 
sublimates will give vivid tints to certain parts of 
the landscape surface, and there must be all the 
more sombre colours which are common to mineral 
matters that have been subjected to fiery influence. 
All these tints will shine and glow with their 
greater or less intrinsic lustres, since they have not 
been deteriorated by atmospheric agencies, and far 
and near they will appear clear alike, since there is 
no aerial medium to veil them or tarnish their 
pristine brightness. 

In the lunar landscape, in the line of sight, there 
are no means of estimating distances; only from an 
eminence, where the intervening ground can be seen, 
is it possible to realise magnitude in a lunar cosmo- 
rama and comprehend the dimensions of the objects 
it includes. 



[To face page 27 ' 2 . 


Plate XXV.—Group of Lunar Mountains (Ideal Lunar Landscape). 










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► 








*4 










■ 

* 




* 



- 














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XIII.] GLOWING LIGHT AN1) BLACK SHADOWS 273 

And with no air there can be no diffusion of 
light. As a consequence, no illumination reaches 
those parts of the scene which do not receive the 
direct solar rays, save the feeble amount reflected 
from contiguous illuminated objects, and a small 
quantity shed by the crescent earth. The shadows 
have an awful blackness. As we stand upon our 
chosen point of observation, we see on the lighted 
side of the rampart almost dazzling brightness, while 
beneath us, on the side away from the sun, there is 
a region many miles in area impenetrable to the 
sight, for there is no object within it receiving 
sufficient light to render it discernible; and all 
around us, far and near, there is the violent contrast 
between intense brightness of insulated parts and 
deep gloom of those in equally intense shadow. 
The black though starlit sky helps the violence of 
this contrast, for the bright mountains in the dis¬ 
tance around us stand forth upon a background 
formed by the darkness of interplanetary space. The 
visible effects of these conditions must be in every 
sense unearthly and truly terrible. The hard, harsh, 
glowing light and pitchy shadows; the absence of 
all the conditions that give tenderness to an earthly 
landscape; the black noonday sky, with the glaring 
sun ghastly in its brightness; the entire absence of 

s 


274 


THE MOON AS A WORLD 


[chap. 


vestiges of any life save that of the long since ex¬ 
pired volcanoes—all these conspire to make up a 
scene of dreary, desolate grandeur that is scarcely 
conceivable by an earthly habitant, and that the 
description we have attempted but insufficiently 
pourtrays. 

A legitimate extension of the imagination leads 
us to impressions of lunar conditions upon other 
senses than that of sight, to which we have hitherto 
confined our fancy. We are met at the outset with 
a difficulty in this extension; for it is impossible to 
conceive the sensations which the absence of an 
atmosphere would produce upon the most import¬ 
ant of our bodily functions. If we would attempt 
the task we must conjure up feelings of suffocation, 
of which the thoughts are, however, too horrible to 
be dwelt upon; we must therefore maintain the 
delusion that we can exist without air, and attempt 
to realise some of the less discomforting effects of 
the absence of this medium. Most notable among 
these are the untempered heat of the direct solar 
rays, and the influence thereof upon the surface 
material upon which we suppose ourselves to stand. 
During a period of over three hundred hours the 
sun pours down his beams with unmitigated ferocity 
upon a soil never sheltered by a cloud or cooled by 


XIII.] EXTREMES OF HEAT AND COLD 


275 


a shower, till that soil is heated, as we have shown, 
to a temperature equal nearly to that of melting 
lead; and this scorching influence is felt by every¬ 
thing upon which the sun shines on the lunar globe. 
But while regions directly isolated are thus heated, 
those parts turned from the sun would remain in¬ 
tensely cold, and that scorching in sunshine and 
freezing in shade with which mountaineers on the 
earth are familiar would be experienced in a terribly 
exaggerated degree. Among the consequences, 
already alluded to, of the alternations of tempera¬ 
ture to which the moon's crust is thus exposed, are 
doubtless more or less considerable expansions and 
contractions of the surface material, and we may 
conceive that a cracking and crumbling of the more 
brittle constituents would ensue, together with a 
grating of contiguous but disconnected masses, and 
an occasional dislocation of them. We refer again 
to these phenomena to remark that if an atmos¬ 
pheric medium existed they would be attended 
with noisy manifestations. There are abundant 
causes for grating and crackling sounds, and such 
are the only sources of noise upon the moon, where 
there is no life to raise a hum, no wind to murmur, 
no ocean to boom and foam, and no brook to plash. 
Yet even these crust-cracking commotions, though 


276 THE MOON AS A WORLD [chap. 

they might be felt by the vibrations of the ground, 
would not manifest themselves audibly, for without 
air there can be no communication between the 
grating or cracking body and the nerves of hearing. 
Dead silence reigns on the moon: a thousand 
cannons might be fired and a thousand drums 
beaten upon that airless world, but no sound could 
come from them: lips might quiver and tongues 
essay to speak, but no action of theirs could break 
the utter silence of the lunar scene. 

At a rate twenty-eight times slower than upon 
earth, the shadows shorten till the sun attains his 
meridian height, and then, from the tropical region 
upon which we have in imagination stood, nothing 
is to be seen on any side, save towards the black 
sky, but dazzling light. The relief of afternoon 
shadow comes but tardily, and the darkness drags 
its slow length along the valleys and creeps slug¬ 
gishly up the mountain-sides till, in a hundred hours 
or more, the time of sunset approaches. This 
phenomenon is but daybreak reversed, and is un¬ 
accompanied by any of the gorgeous sky tints that 
make the kindred event so enrapturing on earth. 
The sun declines towards the dark horizon without 
losing one jot of its brilliancy, and darts the full 
intensity of its heat upon all it shines on to the last. 


XIII.] 


THE SPLENDID EARTH 


277 


Its disc touches the horizon, and in half an hour dips 
half-way beneath it, its intrinsic brightness and 
colour remaining unchanged. The brief interval of 
twilight occurs, as in the morning, when only a 
small chord of the disc is visible, and the long 
shadows now sharpen as the area of light that casts 
them decreases. For a while the zodiacal light vies 
with the earth-moon high in the heavens in illumi¬ 
nating the scene; but in a few hours this solar 
appendage passes out of view, and our world 
becomes the queen of the lunar night. 

At this sunset time the earth, nearly in the 
zenith of us, will be at its half-illuminated phase, 
and even then it will shed more light than we re¬ 
ceive upon the brightest of moonlight nights. As 
the night proceeds, the earth-phase will increase 
through the gibbous stages until at midnight it will 
be “full,” and our orb will be seen in its entire 
beauty. It will perform at least one of its twenty- 
four-hourly rotations during the time that it appears 
quite full, and the whole of its surface features will 
in that time pass before the lunar spectator’s eye. 
At times the northern pole will be tinned towards 
our view, at times the southern; and its polar ice¬ 
caps will appear as bright white spots, marking its 
axis of rotation. If our lunar sojourn were pro- 


278 


THE MOON AS A WORLD 


[chap. 


longed we should observe the northern ice-caps 
creep downwards to lower latitudes (during our 
winter) and retreat again (during our summer); and 
this variation would be perceptible in a less degree 
at the southern pole, on account of the watery area 
surrounding it. The seas would appear (so far as 
can be inferred) of pale blue-green tint; the con¬ 
tinents parti-coloured: and the tinted spots would 
vary with the changing terrestrial seasons, as these 
are indicated by the positions and magnitudes of 
the polar ice-caps. The permanent markings would 
be ever undergoing apparent modification by the 
variations of the white cloud-belts that encircle the 
terrestrial sphere. Of the nature of these variations 
meteorological science is not as yet in a position to 
speak: it would indeed be vastly to the benefit of 
that science if a view of the distribution of clouds 
and vapours over the earth’s surface, as compre¬ 
hensive as that we are imagining, could really be 
obtained. 

It might happen at “ full-earth,” that a black 
spot with a fainter penumbral fringe would appear 
on one side of the illuminated disc and pass some¬ 
what rapidly across it. This would occur when the 
moon passed exactly between the sun and the earth, 
and the shadow of the moon was cast upon the 


mil] APPEARANCE OF THE STARS 279 

terrestrial disc. We need hardly say that these 
shadow transits would occur upon those astronomi¬ 
cally important occasions when an eclipse of the sun 
is beheld from the earth. 

The other features of the sky during the long 
lunar night would not differ greatly from those to 
which we alluded in speaking of its day aspects. 
The stars would be the more brightly visible, from 
the greater power of the eye-pupil to open in the 
absence of the glaring sun, and on this account the 
milky-way would be very conspicuous and the 
brighter nebulae would come into view. The con¬ 
stellations would mark the night by their positions, 
or the hours might be told off (in periods of twenty- 
four each) by the successive reappearances of sur¬ 
face features on certain parts of the terrestrial disc. 
The planets in opposition to the sun would now be 
seen, and a comet might appear to vary the mono¬ 
tony of the long lunar night. But a meteor would 
never flash across the sky, though dark meteoric 
particles and masses would continually bombard 
the lunar surface, sometimes singly, sometimes in 
showers. And these would fall with a compound 
force due to their initial velocity added to that of 
the moon’s attraction. As there is no atmosphere 
to consume the meteors by frictional heat or break 


280 THE MOON AS A WORLD [chap. 

by its resistance the velocity of their descent, they 
must strike the moon with a force to which that of 
a cannon-ball striking a target is feeble indeed. A 
position on the moon would be an unenviable stand¬ 
point from this cause alone. 

The lunar landscape by night needs little descrip¬ 
tion : it would be lit by the earth-moon sufficiently 
to allow salient features, even at a distance, to be 
easily made out, for its moon (i.e., the earth) has 
thirteen times the light-reflecting area that ours has. 
But the night illumination will change in intensity, 
since the earth-moon varies from half-full to full, 
and again to half-full, between sunset and the next 
sunrise. The direction of the light, and hence the 
positions of the shadows, will scarcely alter on 
account of the apparent fixity of the earth in the 
lunar sky. A slight degree of warmth might pos¬ 
sibly be felt with the reflected earth-light; but it 
would be insufficient to mollify the intensity of the 
prevailing cold. The heat accumulated by the 
ground during the three hundred hours’ sunshine 
radiates rapidly into space, there being no atmos¬ 
pheric coat to retain it, and a cooling process 
ensues that goes on till, all warmth having rapidly 
departed, the previously parched soil assumes a 
temperature approaching that of celestial space 


XIII.] 


NIGHT-TIME 


281 


itself, and which has been, as we have stated, 
estimated at between 200° and 250° below the 
Fahrenheit zero. If moisture existed upon the 
moon, its night-side would be bound in a grip of 
frost to which our Arctic regions would be com¬ 
paratively tropical. But since there is no water, the 
aspect of the lunar scenery remains unmodified by 
effects of changing temperature. 

Such, then, are the most prominent effects that 
would manifest themselves to the visual and other 
senses of a being transported to the moon. The 
picture is not on the whole a pleasant one, but it 
is instructive; and our rendering of it, imperfect 
though it be, may serve to suggest other inferences 
that cannot but add to the interest which always 
attaches to the contemplation of natural scenes and 
phenomena from points of view different from those 
which we ordinarily occupy. 


CHAPTEE XIV 


THE MOON AS A SATELLITE : ITS RELATION TO 
THE EARTH AND MAN 

Apart from the recondite functions of the moon 
considered as one of the interdependent members of 
the solar family, into which it would be beyond our 
purpose to inquire, there are certain means by which 
it subserves human interests and ministers to the 
wants of civilised man to which we deem it desir¬ 
able to call attention, especially as some of them 
are not so self-apparent as to have attracted popular 
attention. 

The most generally appreciated because the 
most evident of the uses of the moon is that of a 
luminary. Popular regard for it is usually confined 
to its service in that character, and in that character 
poets and painters have never tired in their efforts 
to glorify it. And obviously this service as a 
“ lesser light ” is sufficiently prominent to excite our 


CHAP. XIV.] 


THE TIDES 


283 


warmest admiration. But moonlight is, from the 
very conditions of its production, of such a change¬ 
able and fugitive nature, and it affords after all so 
partial and imperfect an alleviation of night’s dark¬ 
ness, that we are fain to regard the light-giving 
office of the moon as one of secondary importance. 
Far more valuable to mankind in general, so estim¬ 
able as to lead us to place it foremost in our cate¬ 
gory of lunar offices, is the duty which the moon 
performs in the character of a sanitary agent. We 
can conceive no direful consequences that would 
follow from a withdrawal of the moon’s mere light; 
but it is easy to imagine what highly dangerous 
results would ensue if the moon ceased to produce 
the tides of the ocean. Motion and activity in the 
elements of the terraqueous globe appear to be 
among the prime conditions in creation. Rest and 
stagnation are fraught with mischief. While the 
sun keeps the atmosphere in constant and healthy 
circulation through the agency of the winds, the 
moon performs an analogous service to the waters 
of the sea and the rivers that flow into them. It is 
as the chief producer of the tides—for we must not 
forget that the sun exercises its tidal influences, 
though in much lesser degree—that we ought to 
place the highest value on the services of the moon : 


284 


THE MOON AS A SATELLITE [chap. 


but for its aid as a mighty scavenger, our shores, 
where rivers terminate, would become stagnant 
deltas of fatal corruption. Twice (to speak gener¬ 
ally) a day, however, the organic matter which 
rivers deposit in a decomposing state at their 
embouchures is swept away by the tidal wave ; and 
thus, thanks to the moon, a source of direful pestil¬ 
ence is prevented from arising. Rivers themselves 
are providentially cleansed by the same means, 
where they are polluted by bordering towns and 
cities which, from the nature of things, are sure to 
arise on river banks; and it seems to be also in the 
nature of things that the river traversing a city must 
become its main sewer. The foul additions may be 
carried down by the stream in its natural course 
towards the ocean, but where the river is large 
there will be a decrease in velocity of the current 
near the mouth or where it joins the sea, thus 
causing partial stagnation and consequent deposition 
of the deleterient matters. All this, however, is 
removed, and its inconceivable evils are averted by 
our mighty and ever active “ sanitary commissioner,” 
the moon. We can scarcely doubt that a healthy 
influence of less obvious degree is exerted in the 
wide ocean itself; but, considering merely human 
interests, we cannot suppress the conviction that 


XIV.] TIDAL TRANSPORT 285 

man is more widely and immediately benefited by 
this purifying office of the moon than by any other. 

But the sanitary service is not the only one that 
the moon performs through the agency of the tides. 
There is the work of tidal transport to be con¬ 
sidered. Upon tidal rivers and on certain coasts, 
notwithstanding wind and the use of steam, a very 
large proportion of the heavy merchandise is trans¬ 
ported by that slow but powerful “tug” the flood- 
tide ; and a similar service, for which, however, the 
moon is not to be entirely credited, is done by the 
down-flow of the ebb-tide. Large ships and heavily- 
laden rafts and barges are quietly taken in tow by 
this unobtrusive prime mover, and moved from the 
river's mouth to the far-up city, and from wharf to 
wharf along its banks; and a vast amount of 
mechanical work is thus gratuitously performed 
which, if it had to be provided by artificial means, 
would represent an amount of money value which 
for such a city as London would have to be counted 
by thousands, possibly millions, of pounds yearly. 
For this service we owe the moon the gratitude 
that we ought to feel for a direct pecuniary bene¬ 
factor. 

In the existing state of civilisation and pros¬ 
perity, we do not, however, utilise the power of 


286 THE MOON AS A SATELLITE [chap. 

the tides nearly to the extent of their capabilities. 
Our coal mines, rich with the “ light of other days ” 
—for coal was long ago declared by Stevenson to 
be “ bottled sunshine ”—at present furnish us with 
so abundant a supply of power-generating material 
that in our eagerness to use it upon all possible 
occasions we are losing sight, or putting out of 
mind, many other valuable prime movers, and 
amongst them that of the rise and fall of the 
waters, which can be immediately converted into 
any form of mechanical power by the aid of 
tide-mills. Such mills may be found in existence 
here and there, but for the present they are 
generally outrivalled by the steam engine with 
all its conveniences and adaptabilities; and 
hence they have not shared the benefits of that 
inventive ingenuity which has achieved such 
wonders of mechanical appliance while steam 
has been in the ascendant. But it must be 
remembered that in our extravagant use of 
coal we are drawing from a bank into which 
nothing is being paid. We are consuming an 
exhaustible store, and the time must come when 
it will be needful to look around in quest of 
“ powers that may be.” Then an impetus may 
be given to the application of the tides to 


XIV.] UTILISATION OF THE TIDES 287 

mechanical purposes as a prime mover.* For the 
people of the British Islands the problem would 
have an especial importance, viewing the extent of 
our seaboard and the number of our tidal rivers. 
The source of motion that offers itself is of almost 
incalculable extent. There is not merely the onward 
flowing motions of streams to be utilised, but also 
the lift of water, which, if small in extent, is stu¬ 
pendous in amount; and within certain limits it 
matters little to the mechanician whether the “ foot¬ 
pounds ” of work placed at his disposal are in the 
form of a great mass lifted to a small height or a 
small mass lifted to a great height. There is no 
reason either why the utilisation of the tides should 
be confined to rivers. The sea-side might well 
become the circle of manufacturing industry, and 
the millions of tons of water lifted several feet twice 
daily on our shores might be converted, even by 
schemes already proposed, to furnish the prime 
movement of thousands of factories. And we must 
not forget how completely modern science has 
demonstrated the interconvertibility of all kinds of 
force, and thus opened the way for the introduction 
of systems of transporting power that, in such a 

* About 100 years ago London was supplied with water 
chiefly by pumps worked by tidal mills at London Bridge. 


288 THE MOON AS A SATELLITE [chap. 

state of things as we are for the moment consider¬ 
ing, might be of immense benefit. Gravity, for 
instance, can be converted into electricity; and 
electricity gives us that wonderful power of trans¬ 
mitting force without transmitting (or even moving) 
matter , which power we use in the telegraph, where 
we generate a force at one end of a wire and use it 
to ring bells or deflect needles at the other end, 
which may be thousands of miles away. What we 
do with the slight amount of force needful for tele¬ 
graphy is capable of being done with any greater 
amount. A tide-mill might convert its mechanical 
energy by an electro-magnetic engine, and in the 
form of electricity its force could be conveyed inland 
by proper wires and there reconverted back to 
mechanical or moving power. True, there would be 
a considerable loss of power, but that power would 
cost nothing for its first production. Another means 
ready to hand for transporting power is by com¬ 
pressed air, which has already done good service; 
another is the system so admirably worked out by 
Sir W. [afterwards Lord] Armstrong, of trans¬ 
mitting water-power through the agency of an 
“accumulator,” now so generally used at our 
docks and elsewhere for working cranes and such 
other uses. And as the whole duty of the 


XIV.] THE MARINER'S TIMEKEEPER 289 

engineer is to convert the forces of nature, there is 
a rich field open for his invention, and upon which 
he may one day have to enter, in adapting the pull¬ 
ing force of the moon to his fellow man’s mechanical 
wants through the intermediation of the tides. 

Another of the high functions of the moon is that 
by which she subserves the wants of the navigator, 
and enables him to track his course over the path¬ 
less ocean. Of the two co-ordinates, Latitude and 
Longitude, that are needful to determine the posi¬ 
tion of a ship at sea (or of any standpoint upon the 
earth’s surface) the first is easily found, inasmuch 
as it is always equal to the altitude of the celestial 
pole at the place of observation. But the deter¬ 
mination of the longitude has always been a difficult 
problem, and one upon which a vast amount of in¬ 
genuity has been expended. When it was first 
attacked it was soon discovered that the moon was 
the object of all others by which it could be most 
accurately and, all things considered, most readily 
determined. We must premise that the longitude 
of one place from another is in effect the difference 
between the local times at the two places, so that 
when we say that a place or a ship is, for instance, 
seven hours, twenty-four minutes, ten seconds west 
of Greenwich, we mean that the time-o’-day at the 


290 THE MOON AS A SATELLITE [chap. 

place or ship is seven hours, twenty-four minutes, 
ten seconds earlier than that at Greenwich. Hence, 
finding the longitude at sea or at any place and 
moment means finding what time it is at Greenwich 
at that moment. Of course this could be most 
easily done if we could set a timekeeper at Green¬ 
wich and rely upon its keeping time during a long 
sea voyage ; and this plan appeared so feasible that 
our Government long ago offered a prize of £20,000 
for a timekeeper which would perform to a stated 
degree of accuracy after a certain sea voyage. One 
John Harrison did make such a timekeeper, that 
actually satisfied the conditions, and obtained the 
prize: and chronometers are now largely used for 
longitude, their construction having been brought 
to great perfection, especially in England, owing to 
a continuance (in a less liberal degree, however) of 
Government inducement. But chronometers are 
not entirely to be relied on, even where several are 
carried, which in other than Government ships is 
rarely the case: recourse must be had to the 
heavenly bodies for check upon the timekeeper. 
And the moon is, as we have said, the body that 
best serves the requirements of the problem. 

The lunar method for longitude amounts practi¬ 
cally to this. The stars are fixed; the sun, moon, 


XIV.] the METHOD OF WORKING 291 

and planets move amongst them; the sun and 
planets with very slow rates of apparent motion, 
the moon with a very rapid one. If, then, it be 
predicted that at a certain instant of Greenwich 
time the moon will be a certain distance from a 
fixed star, and if the mariner at sea observes when 
the moon has that exact distance, he will know the 
Greenwich time at the instant of his observation.* 
The moon thus becomes to him as the hand of a 
timepiece, whereof the stars are the hour and minute 
marks, the whole being, as it were, set to Greenwich 
time. The requisite predictions of the distance (as 
seen from the earth’s centre) of the moon from con¬ 
venient fixed stars, or from the sun, or any of the 
principal planets—whose calculated places are so 
accurate that they may for this purpose be used as 
fixed stars—are given to the utmost exactness in 
the navigators’ vade mecum , the Nautical Almanac 
for every third hour, day and night, of Greenwich 
time (except for a few days near new-moon, when 
the moon cannot be seen); and from these given 

* The sun and planets are comparatively useless for this 
object, because of their slow movement among the stars; the 
change of their positions from hour to hour is so small as to 
render uncertain the Greenwich times deducible therefrom. 
Their use would be comparable to taking the time from the 
hour-hand of a clock. 


292 


THE MOON AS A SATELLITE 


[chap. 


distances the navigator can, by a simple process of 
differencing, obtain the Greenwich time correspond¬ 
ing to the distance which he may have observed. * 
Then knowing, as he does by other observations 
easily obtained, the local or ship’s time of his 
observation, he takes the difference between this 
and the corresponding Greenwich time, and this 
difference is his longitude from Greenwich. Of 
course the whole value of this method depends 
upon the exactitude of the predicted distances 
corresponding to the given Greenwich times. These 
distances are obtained by tables of the moon’s 
motions, which must be found from observations. 
The motions in question are of an intricacy almost 
past comprehension, on account of the disturbing 
forces to which the moon is subjected by the sun 
and planets. The powers of the profoundest 
mathematicians, from Newton downwards, have 
been severely exercised in efforts to group them into 
a theory, and represent them by tables capable of 
furnishing the requisite exact predictions of lunar 
positions for nautical purposes. Accurate observa¬ 
tions of the moon’s place night after night have, 

* Certain corrections are necessary to clear his observed 
distance of the effects of parallax and refraction; upon these, 
however, we cannot enter here. 


XIV.] THE MOON AND THE MONTHS 293 

from the dawn of this lunar method for longitude, 
been in urgent request by mathematicians for the 
purposes specified, and it was solely to procure 
these observations that the Observatory at Green¬ 
wich was established, and mainly for their continued 
prosecution (and for the stellar observations neces¬ 
sary for their utilisation) that it is sustained. For 
two centuries the moon has been unremittingly 
observed at Greenwich, and the tables at present 
used for making the Nautical Almanac (those 
formed by Prof. Hansen) depend upon the observa¬ 
tions there obtained. The work still goes on, for 
even now the degree of exactitude is not what is 
desired, and astronomers are looking forward with 
some interest to new lunar tables which were left 
complete by the late M. Delaunay, formerly the 
head of astronomy in France, based upon a theory 
which he evolved. This use of the moon is the 
grandest of all in respect of the results to which it 
has led. 

Then, too, regarding the moon as a timekeeper, 
we must not forget the service that it renders in 
furnishing a division of time intermediate between 
the day—which is measured by the earth’s rota¬ 
tion—and the year, which is defined by the earth’s 
orbital revolution. Notwithstanding the survival of 


294 THE MOON AS A SATELLITE [chap. 

lunar reckoning in our religious services, we, in our 
time and country, scarcely need a moon to mark 
our months ; but we must not forget that with many 
ancient people the moon was, and with some is still, 
the chief timekeeper, the calendars of such people 
being lunar ones, and all their events being reckoned 
and dated by “moons.” To us, however, the moon 
is of great service in this department by enabling us 
to fix dates to many historical events, the times of 
occurrence of which are uncertain, by reason of 
defective records or by dependence upon such un¬ 
certain data as “lives of emperors,” years of this or 
that king’s reign, or generations of one or another 
family. The moon now and then clears up a 
mystery, or decides a disputed point in chronology, 
by furnishing the accurate date of an ancient eclipse, 
which was a phenomenon that always inspired awe 
and secured for itself careful record. The chrono- 
loger is continually applying to the astronomer for 
the date and place of visibility of some total eclipse, 
of which he has found an imperfect record, veritable 
as to the fact, but dated only by reference to some 
year of a so-and-so’s reign, or by some battle or 
other historical occurrence. The eclipses that 
occurred near the time are then examined, and 
when one is found that tallies with recorded condi- 


XIV.] 


HISTORICAL ECLIPSES 


295 


tions in other respects (such as the time of day and 
the place of observation), its indisputable date 
becomes a starting-point from which the chrono- 
loger works backwards and forwards in safety. 
There is one famous eclipse—that predicted by 
Thales six centuries before Christ, which put an end 
to the battle between the Medes and Lydians by 
the terror its darkness created in both armies— 
which is most intimately associated with ancient 
chronology, and has been used to rectify a proxi¬ 
mate date (the first year of Cyrus of Babylon) which 
forms the foundation of all Scripture chronology. 
Sacred and profane history alike are continually 
receiving assistance from the accurate dates which 
the moon, by having caused eclipses of the sun, 
enables the astronomer to fix beyond cavil or doubt. 

The mention of eclipses reminds us, too, of the 
use which the moon has been in increasing, through 
them, our knowledge of the physical condition of 
the sun. If the moon had never intervened to cut 
off the blinding glare of the solar disc, we should 
have been to this day left to assume that the sun 
is all-contained by the dazzling globe that we 
ordinarily see. But, thanks to the moon’s inter¬ 
vention, we now know that the sun is by no means 
the mere naked sphere we should have suspected. 


296 THE MOON AS A SATELLITE [chap. 

Eclipses have taught us that it is surrounded by an 
envelope of glowing gases, and that it has a vast 
vaporous surrounding, beyond its glowing atmos¬ 
phere, which appears to be composed of matter 
streaming away from the sun into surrounding 
space. With these discoveries still in their infancy, 
it is impossible to foresee the knowledge to which 
they will eventually lead, but they can hardly be 
barren of fruit, and whatever they ultimately teach 
will be so much insight gained into the sublimest 
problem that human science has before it — the 
determination of the source and maintaining power 
of the light and heat and vivifying agency of the sun. 
In according our thankful reflections to the moon 
for these revelations, we must not forget that, 
should there be inhabitants upon our neighbouring 
worlds, Mercury, Venus, and Mars, which have no 
satellites, they, the supposed inhabitants, can gain 
no such knowledge upon the surroundings of the 
ruler of the solar system. On the other hand, any 
rational being who may be supposed to dwell upon 
Saturn or Jupiter, would, through the intervention 
of their numerous moons, have, in the latter case 
especially, far more abundant opportunities of ac¬ 
quiring the knowledge in question than we have. 

Finally, there is a use of the moon which touches 


XIV.] A “ MEDAL OF CREATION ” 297 

us, author and reader, very closely. It has taught 
us of a world in a condition totally different from 
our own; of a planet without water, without air, 
without the essentials to life development, but 
rather with the conditions for life destruction; a 
planet left by the Creator—for wise purposes that 
we cannot fully know—as it were but half-formed, 
with all the igneous foundations fresh from the 
cosmical fire, and with its rough-cast surface in its 
original state, its fire and mould-marks exposed to 
our view. From these we have essayed to resolve 
some of the processes of formation, and thus to 
learn something of the cosmical agencies that are 
called forth in the purely igneous era of a planet’s 
history. We trust that we, on our part, have shown 
that the study of the moon may be a benefit not 
merely to the astronomer, but to the geologist; 
for we behold in it a mighty “medal of creation” 
doubtless formed of the same material and struck 
with the same die that moulded our earth; but 
while the dust of countless ages and the action of 
powerful disintegrating and denuding elements have 
eroded and obliterated the earthly impression, the 
superscriptions on the lunar surface have remained 
with their pristine clearness unsullied, every vestige 
sharp and bright as when it left the Almighty 


[CHAP. 


298 THE MOON AS A SATELLITE 

Maker’s hands. The moon serves no second-rate 
or insignificant service when it teaches us of the 
variety of creative design in the worlds of our system, 
and exalts our estimation of this peopled globe of 
ours by showing us that all the planetary worlds 
have not been deemed worthy to become the habi¬ 
tations of intelligent beings. 

Reflections upon the uses of the moon not un¬ 
naturally lead our thoughts to some matters that 
may be regarded as abuses. These mainly take the 
form of superstitions, erroneous beliefs in the moon’s 
influence over terrestrial conditions, and occasionally 
of erroneous ideas upon the moon’s functions as a 
luminary. The first-mentioned are almost beneath 
notice, for they include such mythical suspicions as 
that the moon influences human sanity and other 
affections of mind and body; that the moon’s rays 
have a decomposing effect upon organic matter; 
that they produce blindness by shining upon a 
sleeper’s eyes; that the moon determines the hours 
of human death, which is supposed to occur with 
the change of the tide, etc. All such, having no 
foundation on fact, are put beyond our considera¬ 
tion. The third matter we have mentioned may 
also be dismissed in a very few words. The errone- 


XIV.] 


SOME ERRORS 


299 


ous ideas upon the moon’s functions as a luminary, 
to which we allude, are those which are manifested 
by poets and painters, and even historians, who do 
not hesitate to bring the moon upon a scene in any 
form and at any time they please without reference 
to actual lunar circumstances. It is no uncommon 
thing to see, in a picture representing an evening 
scene, a moon introduced which can only be seen in 
the morning—a waning moon instead of a waxing 
one; and astronomical critics have, indeed, caught 
artists so far tripping as to put a moon in a picture 
representing some event that occurred upon a date 
when the moon was new, and therefore invisible. 
Writers take the same liberties very frequently. 
A newspaper correspondent, during the Franco- 
Prussian war, described the full moon as shining 
upon a scene of desolation on a particular night, 
when really there was no moon to be seen. One of 
the most flagrant cases of this kind, however, occurs 
in Wolfe’s ballad on “The death of Sir John 
Moore,” where it is written that the hero was buried 
“ By the struggling moonbeam’s misty light.” But 
the interment actually took place at a time when 
the moon was out of sight. We mention these 
abuses of the moon in the hope of promoting a 
better observance of the moon’s luminary office. 


300 THE MOON AS A SATELLITE [chap. 

They who wish to bring the moon upon a scene, not 
knowing ipso facto that it was there, should take the 
advice of Nick Bottom in the “ Midsummer Night's 
Dream,” and make sure of their object by consult¬ 
ing an almanac. 

The second of the specified abuses to which the 
moon is subject refers to its supposed influence on 
the weather; and in the extent to which it goes this 
is one of the most deeply rooted of popular errors. 
That there is an infinitesimal influence exerted by 
the moon on our atmosphere will be seen from the 
evidence we have to offer, but it is of a character 
and extent vastly different from what is commonly 
believed. The popular error is shown in its most 
absurd form when the mere aspect of the moon, the 
mere transition from one phase of illumination to 
another, is asserted to be productive of a change of 
weather; as if the gradual passage from first quarter 
to second quarter, or from that to third, could of 
itself upset an existing condition of the atmosphere ; 
or as if the conjunction of the moon with the sun 
could invert the order of the winds, generate clouds, 
and pour down rains. A moment’s reasoning ought 
to show that the supposed cause and the observed 
effect have no necessary connection. In our climate 
the weather may be said to change at least every 


XIV.] THE MOON AND THE WEATHER 301 

three days, and the moon changes—to retain the 
popular term—every seven days; so that the proba¬ 
bility of a coincidence of these changes is very great 
indeed: when it occurs, the moon is sure to be 
credited with causing it. But a theory of this kind 
is of no use unless it can be shown to apply in every 
case; and, moreover, the change must always be in 
the same direction; to suppose that the moon can 
turn a fine day to a wet one, and a wet day to a fine 
morrow indiscriminately, is to make our satellite 
blow hot and cold with the same mouth, and so to re¬ 
duce the supposition to an absurdity. If any marked 
connection existed between the state of the air 
and the aspect of the moon, it must inevitably have 
forced itself unsought upon the attention of meteoro¬ 
logists. In the weekly return of Births, Deaths, 
and Marriages, issued by the Registrar-General, a 
table is given showing all the meteorological ele¬ 
ments at Greenwich for every day of the year, and 
a column is set apart for noting the changes and 
positions of the moon. These reports extend back¬ 
wards nearly a quarter of a century. Here, then, 
is a repertory of data that ought to reveal at a glance 
any such connection, and would certainly have done 
so had it existed. But no constant relation between 
the moon columns and those containing the instru- 


302 THE MOON AS A SATELLITE [chap. 

merit readings has ever been traced. Our meteoro¬ 
logical observatories furnish continuous and un¬ 
broken records of atmospheric variations, extending 
over long series of years: these afford still more 
abundant means for testing the validity of the lunar 
hypothesis. The collation has frequently been made 
for special points in the inquiry, and certainly some 
connection has been found to obtain between 
certain positions of the moon in her orbit and 
certain instrumental averages; but so small are the 
effects traceable to lunar influence, that they are 
almost inappreciable among the grosser irregu¬ 
larities that arise from other and as yet unexplained 
causes. 

The lunar influences upon our atmosphere most 
likely to be detected are those of a tidal character, 
and those due to the radiation of the heat which the 
moon receives from the sun. The first would be 
shown by the barometer, which may be called an 
“ atmospheric tide gauge.” Some years ago Colonel 
Sir Edward Sabine instituted a series of observa¬ 
tions at St Helena, to determine the variations of 
barometric indications from hour to hour of the 
lunar day. The greatest differences were found to 
occur between the times when the moon was on the 
meridian, and when it was six hours away from the 


XIV.] HEAT FROM THE MOON 303 

meridian; in other words, between atmospheric 
high tide and low tide. But the average of these 
differences amounted only to the four-hundredth 
part of an inch on the instrument’s scale; a quantity 
that no weather observer would heed, that none but 
the best barometers would show, and that can have 
no perceptible effect on weather changes. The 
distance of the moon from the earth varies, as is 
well known, in consequence of the elliptical form of 
her orbit: this variation ought also to produce an 
effect upon the instrument’s indications ; but Colonel 
Sabine’s analysis showed that it was next to in¬ 
sensible ; the mean reading at apogee differing from 
that at perigee by only the two-thousandth part of 
an inch. Schubler, a German meteorologist, had 
arrived at similarly negative results some years pre¬ 
viously. Hence it appears that the great index of 
the weather is not sensibly affected by the state of 
the moon : the conclusion to be drawn with regard 
to the weather itself is obvious enough. As regards 
the heat received from the moon, we know, from 
the recent experiments of Lord Eosse in England, 
and Marie Davy in France, elsewhere alluded to, 
that a degree of warmth appreciable to the highly 
sensitive thermophile is exerted by the moon upon 
the earth near to the time of full moon, when the 


304 THE MOON AS A SATELLITE [chap. 

sun’s rays have been pouring their unmitigated heat 
upon the lunar surface continuously for fourteen 
days. And as it is improbable that the whole of 
the heat sent earthwards from the moon reaches the 
earth’s surface, we must infer that a considerable 
amount is absorbed in the higher atmosphere, and 
does work in evaporating the lighter clouds and 
thinning the denser ones. The effect of this upon 
the earth is to facilitate the radiation of its heat into 
space, and so to cool the lower atmospheric strata. 
And this effect has been shown to be a veritable 
one by an exhaustive tabulation of temperature 
records from various observatories, which was 
undertaken by Mr Park Harrison. The general 
conclusion from these was, that the temperature at 
the earth’s surface is lower by about 2i degrees at 
moon’s last quarter than at first quarter; the para¬ 
doxical result being what would naturally follow 
from the foregoing consideration. The tendency of 
the full moon to clear the sky has been remarked 
by several distinguished authorities, to wit, Sir 
John Herschel, Humboldt, and Arago; and in 
general the clearing may be accepted as a meteoro¬ 
logical fact, though in one case of close examination 
it has been negatived. It cannot be doubted that a 
full moon sometimes shows a night to be clear that 


XIV.] POPULAR DELUSION 305 

would in the absence of the moon be called 
cloudy. 

When close comparisons are made between the 
moon’s positions and records of rain-fall and wind- 
direction, dim indications of relation exhibit them¬ 
selves, which may be the feeble consequences of the 
change of temperature just spoken of; but in every 
case where an effect has been traced it has been of 
the most insignificant kind, and no apparent con¬ 
nection has been recognised between one effect and 
another. Certainly there is nothing that can support 
the extensive popular belief in lunar influence on 
weather, and nothing that can modify the conviction 
that this belief as at present maintained is an absurd 
delusion. Yet its acceptance is so general, and 
runs through such varied grades of society, that we 
have felt it our duty to dwell upon it to the extent 
that we have done. 


u 


CHAPTER XV 


CONCLUDING SUMMARY 

Having arrived at the conclusion of our subject, it 
appears to us desirable that we should recall to the 
reader, by a rapid review, its salient features. 

Our main object being to attempt what we con¬ 
ceive to be a rational explanation of the surface 
details of the moon which should be in accordance 
with the generally received theory of planetary for¬ 
mation, and with the peculiar physical conditions of 
the lunar globe—the opening of our work was a 
summary of the nebular hypothesis as it was started 
by the first Herschel and systemised by Laplace. 
Following these philosophers we endeavoured to 
show how a chaotic mass of primordial matter 
existing in space would, under the action of gravita¬ 
tion, become transformed into a system of planetary 
bodies circulating about a common centre of gravity; 
and further, how, in some cases, the circulating 

306 


CHAP. XV.] the CREATION OF THE MOON 307 

planetary masses would themselves become sub¬ 
centres of satellite systems; our earth being one of 
these sub-centres with only one satellite attendant 
— to wit, the moon, the subject of our study. 

The moon being thus considered as evolved 
from the parent nebulous mass, and existing as an 
isolated and compact body, we had next to consider 
what was the effect of the continued action of the 
gravitating force. By the light of the beautiful 
“ mechanical theory of heat ” we argued that this 
force, not being destructible , but being convertible , 
was turned into heat; and that whatever may have 
been the original condition of the parent nebulous 
mass, as regards temperature, its planetary offspring 
became elevated to an intense degree of heat as 
they assumed the form of spheres under the influ¬ 
ence of gravitation. 

The incandescent sphere having attained its 
maximum degree of heat by the total conversion 
thereinto of the gravitating force it embodied, we 
explained how there must have ensued a dispersion 
of that heat by radiation into surrounding space, 
resulting in the cooling and consequent solidification 
of the outermost stratum of the lunar sphere, and 
subsequently in the continuation of the cooling pro¬ 
cess downwards or inwards to the centre. And 


308 


CONCLUDING SUMMARY 


[chap. 


here we essayed to prove that in this second stage 
of the cooling process, when the crust was solid and 
the subjacent portion of the molten sphere was 
about to solidify, there would come into operation a 
principle which appears to govern the behaviour 
of certain fusible substances, and which may be 
concisely termed the principle of pre-solidifying 
expansion. We adduced several examples of the 
manifestation of this principle, soliciting for it the 
careful consideration of physicists and geologists, 
and looking to it as furnishing the key to the 
mystery of volcanic action upon the moon, since, 
without needing recourse to aqueous or gaseous 
sources of eruptive power, it afforded a rationale of 
the ejection of the fluid and semi-fluid matter of the 
moon through the solidified crust thereof, and also 
of the dislocations of that crust, unattended by 
actual ejection of sub-surface matter, of which our 
satellite presents a variety of examples, and which 
the earth also appears to have experienced at some 
period of its formative history. 

Arrived at this stage of our subject we thought 
it needful to introduce some pages of data and 
descriptive detail. Accordingly in one chapter we 
discussed the form, magnitude, weight, and density 
of the moon, and the force of gravity at its surface : 


XV.] 


THE MOON’S TOPOGRAPHY 


309 


and the more soundly to fix these data in the mind, 
we devoted a few lines to explanation of the methods 
whereby each has been ascertained. We then ex¬ 
amined the question (so important to our subject) of 
the existence or non-existence of a lunar atmo¬ 
sphere, giving the evidence, which may be regarded 
as conclusive, in proof of the absence of both air and 
water from the moon, and, therefore, refuting the 
claim of these elements to be considered as sources 
or influencers of the moon’s volcanic manifestations. 
A general coup d’oeil of the lunar hemisphere facing 
the earth next engaged our attention, and we con¬ 
sidered the aspect of the disc as it is viewed by the 
naked eye and with telescopes of various powers. 
From this general survey we passed to the topog¬ 
raphy of the moon, tracing briefly the admirable 
labours of those who have advanced this subject, 
and, by aid of picture and skeleton maps, placing it 
within the reader’s power to become more than 
sufficiently acquainted for the purposes of this work 
with the names and positions of detailed objects and 
features of interest. Special descriptions of in¬ 
teresting and typical spots and regions were given 
in some few cases where such appeared to be called 
for. 

These descriptive matters disposed of, we pro- 


310 


CONCLUDING SUMMARY 


[chap. 


ceeded to discuss the various classes of surface 
features with a view to explaining the precise 
actions which appear to us to have led to their for¬ 
mation. Naturally the craters first demanded our 
attention. We pointed out the reasons for regard¬ 
ing the great majority of the circular formations of 
the moon as craters, as truly volcanic as those of 
which we have examples, modified by obvious 
causes, upon the earth; and, tracing the causative 
phenomena of terrestrial volcanoes, we showed how 
the explanations which have been offered to account 
for them scarcely apply to those of the moon : N and 
thus, driven to other hypotheses, we endeavoured to 
demonstrate the probability of the lunar craters 
having been produced by eruptive force, generated 
by that pre-solidifying expansion of successive por¬ 
tions of the moon’s molten interior, which we 
enunciated in our third chapter. The precise cause 
of phenomena which resulted in the production of a 
crater of the normal lunar type, with or without the 
significant central cone, were then illustrated by a 
series of step-by-step diagrams with accompanying 
descriptive paragraphs. And after treating of 
craters of the normal type we pointed out and 
explained some variations thereupon that are here 
and there to be met with, and likewise those curious 


XV.] CRATERS AND MOUNTAIN RANGES 311 

complications of arrangement which exhibit craters 
superimposed one upon another and intermingled in 
strange confusion. 

From craters manifestly volcanic we passed to 
the consideration of those circular formations which, 
from their vastness of size, scarcely admit of satis¬ 
factory explanation by a volcanic hypothesis.^ We 
summarised several proffered theories of their origin, 
and pointed out what we considered might be a 
possible key to the solution of the selenological 
enigma which they constitute, without, however, 
expressing ourselves entirely satisfied with the 
validity of our suggestion. The less mysterious 
features presented by peaks and mountain ranges 
were then discussed to the extent that we considered 
requisite, viewing their comparatively simple char¬ 
acter and the secondary position they occupy in 
point of numerical importance upon the moon. At 
greater length we dealt with the cracks and chasms 
and the allied phenomena of radiating streaks, 
pointing out with regard to these latter the strik¬ 
ingly beautiful correspondence in effect (and there¬ 
fore presumably in cause) between them and crack- 
systems of a glass globe “ starred ” by an expanding 
internal medium. 

The more notable objects and features of the 


312 


CONCLUDING SUMMARY 


[chap. 


lunar surface being disposed of, we had next to say 
a few words upon some residual phenomena, chiefly 
upon the colour of lunar surface details, and upon 
their various degrees of brightness or reflective 
power. And, inasmuch as varying brightness 
seemed to us to be related to varying antiquity, we 
were thence led to the question of the chronology of 
selenological formations, and to the disputation 
upon the continuance of volcanic action upon the 
moon in recent years. We regarded this question 
from the observational and the inferential points of 
view, and were led to the conclusion that the moon’s 
surface arrived at its terminal condition ages ago, 
and that it is next to hopeless to look for evidence 
of existing change. 

Thus far our work dealt with the moon as a 
planetary body merely. It occurred to us, however, 
that we might add to the interest attaching to our 
satellite were we to regard it for a time as a world, 
and consider its conditions as respects fitness for 
habitation by beings like ourselves. The arguments 
against the possibility of the moon being thus fitted 
for human creatures, or, indeed, for any high 
organism, were decisive enough to require little 
enforcing. It appeared to us, nevertheless, that 
much might be learnt by imagining one’s self located 


XV.] 


THE MOON’S UTILITY 


313 


upon the moon during a period embracing one lunar 
day (a month of our reckoning), with power to 
comprehend the peculiar circumstances and con¬ 
ditions of such a situation. We therefore attempted 
a description of an imaginary sojourn upon the 
moon, and pointed out some of the more striking 
aspects and phenomena which we know by legiti¬ 
mate inference would be there manifested. We 
trust, that while our modest efforts in the chapter 
referring to this branch of our subject may prove 
in some degree entertaining, they may be in a 
greater degree instructive, inasmuch as certain facts 
are brought into prominence which would not un¬ 
naturally be overlooked in contemplating the moon 
from the earth, the only real standpoint that is 
available to us. 

In our final chapter we considered the moon as 
a satellite, and sought to enhance popular regard for 
it on account of certain high functions which it per¬ 
forms for man’s benefit on this earth; but which 
are in great risk of being overlooked. We showed 
that, notwithstanding the moon’s occasionally useful 
service as a nocturnal luminary, it fills a far higher 
office as a sanitary agent by cleansing the shores of 
our seas and rivers through the agency of the tides. 
We pointed out the vast amount of absolutely 


314 CONCLUDING SUMMARY [chap. 

mechanical work and commercial labour which the 
same tidal agency executes in transporting mer¬ 
chandise up and down our rivers—an amount that, 
to take the port of London alone, represents a 
money value per annum that may be reckoned in 
millions sterling, seeing that if our river was tideless 
all transport would have to be done by manual or 
steam power. We then hinted at the stupendous 
reservoir of power that the tidal waters constitute, 
a form of power which has not as yet been suffi¬ 
ciently called into operation, but which may be 
invoked by-and-by, when we have begun to feel 
more acutely the consequences of our present pro¬ 
digal use of the fuel that was stored up for us by 
bountiful nature ages upon ages ago. The moon’s 
services to the navigator, in affording him a ready 
means of finding his longitude at sea; to the 
chronologist and historian, as a timekeeper, counting 
periods too vast for accurate reckoning by other 
means; to the astronomer and student of nature, in 
revealing certain wonderful surroundings of the 
solar globe, which, but for the phenomena of 
eclipses caused by the moon’s interposition, would 
never have been suspected to exist—these were 
other functions that we dwelt upon, all too briefly 
for their deserts ; and, lastly, we spoke of the moon 


XV.] 


CONCLUSION 


►15 


as a medal of creation fraught with instructive sug¬ 
gestions, which it has been our endeavour to bring 
to notice in the course of this work. And from uses 
we passed to abuses, directing attention to a few 
popular errors and wide-spread illusions relating to 
lunar influence upon, and in connection with things 
terrestrial. This part of our work might have been 
considerably expanded, for, in truth, the moon has 
been a misunderstood and misjudged body. Some 
justice we trust we have done to her: we have 
brought her face to the fireside; we have analysed 
her features, and told of virtues that few of her 
admiring beholders conceived her to possess. We 
have traced out her history, fraught with wonderful 
interest, and doubtless typical of the history of other 
spheres that in countless numbers pervade the 
universe: and now, having done our best to make 
all these points familiar, we commend the moon to 
still further study and still more intimate acquaint¬ 
ance, confident that she will repay all attentions, be 
they addressed to her as 

A PLANET, A WORLD, OR A SATELLITE, 


OLIVER AND BOYD 
PRINTERS 
EDINBURGH 







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